Showing papers in "Ices Journal of Marine Science in 2004"

Microzooplankton production in the oceans

Abstract: A literature synthesis of phytoplankton growth (m) and grazing (m) rate estimates from dilution experiments reveals that microzooplankton account for most phytoplankton mortality in the oceans, averaging 60e75% of daily phytoplankton production (PP) across a spectrum of open-ocean and coastal systems. For reasonable estimates of gross growth efficiency (GGEZ 30e40%), such impacts imply that secondary production rates of microzooplankton (MP2() are typically in the range 21e34% of PP. However, multiple trophic transfers within the microbial community can further enhance total microzooplankton production by an additional third to a half (MPtot Z 28e55% of PP). These estimates are 2e5 times typical values for bacterial production (10e15% of PP). Thus, in aggregate and on average, microzooplankton consume substantially more (6e7 times) production from phytoplankton than from heterotrophic bacteria. High grazing impacts and relatively high GGEs are consistent with population growth rates for microzooplankton and phytoplankton that are roughly equivalent under ambient conditions, which may be requisite for grazing regulation. Transfer efficiencies of microzooplankton production to mesozooplankton depend critically on the number of predatory interactions among microconsumers, and may be one way in which systems differ substantially. Overall, the ability to quantify microzooplankton production in terms of more broadly measured rates of PP provides a potential avenue for broadening our understanding of ocean community dynamics through remote sensing and modelling.

Enumeration, measurement, and identification of net zooplankton samples using the ZOOSCAN digital imaging system

Abstract: Grosjean, P., Picheral, M., Warembourg, C., and Gorsky, G. 2004. Enumeration,measurement, and identification of net zooplankton samples using the ZOOSCAN digitalimaging system. e ICES Journal of Marine Science, 61: 518e525.Identifying and counting zooplankton are labour-intensive and time-consuming processesthat are still performed manually. However, a new system, known as ZOOSCAN, has beendesigned for counting zooplankton net samples. We describe image-processing and theresults of (semi)-automatic identification of taxa with various machine-learning methods.Each scan contains between 1500 and 2000 individuals !0.5 mm. We used two trainingsets of about 1000 objects each divided into 8 (simplified) and 29 groups (detailed),respectively. The new discriminant vector forest algorithm, which is one of the mostefficient methods, discriminates between the organisms in the detailed training set with anaccuracy of 75% at a speed of 2000 items per second. A supplementary algorithm tagsobjects that the method classified with low accuracy (suspect items), such that they could bechecked by taxonomists. This complementary and interactive semi-automatic processcombines both computer speed and the ability to detect variations in proportions and greylevels with the human skills to discriminate animals on the basis of small details, such aspresence/absence or number of appendages. After this checking process, total accuracyincreases to between 80% and 85%. We discuss the potential of the system as a standard foridentification, enumeration, and size frequency distribution of net-collected zooplankton.

Stock identification of Mediterranean horse mackerel (Trachurus mediterraneus) using morphometric and meristic characters

Abstract: Turan, C. 2004. Stock identification of Mediterranean horse mackerel (Trachurusmediterraneus) using morphometric and meristic characters. e ICES Journal of MarineScience, 61: 774e781.Morphologic differentiation among stocks of Mediterranean horse mackerel, Trachurusmediterraneus, throughout the Black, Marmara, Aegean and Eastern Mediterranean Seas,was investigated using morphometric and meristic characters. Discriminant functionanalysis of both morphometric and meristic characters suggested that there is restrictedmigration of mackerel among the adjacent seas. Overlapping of four Black sea samples onthe discriminant space in morphometric and meristic characters suggested that there is oneself-recruiting population in the area. The Marmara sea samples were the most isolatedsamples from all others for both morphometric and meristic characters, which may indicateexistence of a distinguishable mackerel stock in the area. The sample from the Aegean Seawas grouped with one geographically close Mediterranean sample based on morphometrics,and separated from all other Mediterranean samples based on meristic characters,suggesting some degree of intermingling between these areas. Examination of thecontribution of each morphometric variable to canonical functions indicated that differencesamong samples seemed to be associated with the anterior part of the body. In meristicanalyses, highest contributions to canonical functions were associated with the number ofgill rakers and pectoral fin rays.

Testing candidate indicators to support ecosystem-based management: The power of monitoring surveys to detect temporal trends in fish community metrics

Abstract: Community metrics describe aspects of community structure and are often calculated from species-size-abundance data collected during fish stock monitoring surveys. Several community metrics have been proposed as indicators to support ecosystem-based fishery management. These metrics should be sensitive to fishing impacts and respond rapidly to management action, so that managers can assess whether changes in the fish community are a desirable or undesirable response to management. It should also be possible to estimate metrics with sufficient precision so that changes in the community can be detected on management time scales of a year to a few years. Here, we test the power of a large-scale annual trawl survey (North Sea International Bottom Trawl Survey, IBTS) to detect trends in six community metrics: mean length, mean weight, mean maximum length, mean maximum weight, slope of the biomass size spectrum, and mean trophic level. Our analyses show that the power of the trawl survey to detect trends is generally poor. While community metrics do provide good long-term indicators of changes in fish community structure, they are unlikely to provide an appropriate tool to support short-term management decisions. If fish community metrics are to provide effective support for ecosystem-based management, and management time scales cannot be extended, then the power of many surveys to detect trends in fish community structure will need to be improved by increased replication and standardization.

Feeding preferences of herring (Clupea harengus) and sprat (Sprattus sprattus) in the southern Baltic Sea

Abstract: No field studies have been performed on the selectivity of herring and sprat in the southern Baltic Sea in relation to their entire range of prey. Accordingly, we tested in the field the following hypotheses: (i) sprat and herring are selective feeders and (ii) sprat and herring selectivity is size- and season- dependent. The results show that (i) smaller herring and all size classes of sprat are strictly zooplanktivorous, selecting principally Temora longicornis and Bosmina maritima during the autumn and Pseudocalanus elongatus in winter; (ii) larger herring are essentially nektobenthos feeders, predating on Mysis mixta during the autumn and amphipods and polychaetes during the winter; and (iii) herring and sprat seem to avoid Acartia spp. in both autumn and winter. During the autumn, herring are zooplanktivorous up to 18e20 cm, whereas in winter herring feed on nektobenthos starting from 14e15 cm. Selectivity was not an absolute process but it was related to prey relative abundance in the sea and, possibly, to prey profitability (e.g. size, conspicuousness, and reaction time).

Daily egg production of anchovy in European waters

Abstract: Since the late 1980s, the Daily Egg Production Method (DEPM) has been applied to several anchovy stocks in European waters. DEPM surveys in the Bay of Biscay were well standardized and focused on providing fisheries-independent information for stock assessment purposes. Those targeting Mediterranean stocks were largely experimental and often opportunistic, with the main aim of developing and testing the method, rather than providing estimates of spawning stock biomass (SSB) for stock assessment. Consequently, the DEPM has been applied once, twice, or a maximum of three times in certain Mediterranean areas with no among-area standardization. Different techniques for several aspects of the method have been used in the Mediterranean, and the parameters estimated vary greatly among stocks and year of application. Evidence is provided that variability in biological production among sub-basins and/or years, a characteristic of Mediterranean Sea, may directly affect anchovy egg production. The daily specific fecundity of anchovy stocks can vary greatly among years, areas, or seasons in response to changing environmental and trophic regimes. When the correlation between regression-derived estimates of daily egg production and associated estimates of daily specific fecundity for anchovy in the Mediterranean, the Bay of Biscay, and upwelling areas are compared, a significant isometric relationship emerges for the Mediterranean and the Bay of Biscay, implying density-dependent use of spawning habitat. In upwelling areas, estimates of daily egg production are relatively high for a narrow range of generally low daily specific fecundities. There is a strong linear relationship between anchovy SSB and spawning area in European waters that does not differ significantly between the Bay of Biscay and the Mediterranean Sea.

Temperature-dependent fractionation of stable oxygen isotopes in otoliths of juvenile cod (Gadus morhua L.)

Abstract: Analysis of stable oxygen isotopes in otoliths is a promising technique for estimating the ambient temperature experienced by fish, but consistent equations relating temperature and fractionation of stable oxygen isotopes in otoliths among different fish species are lacking. Juvenile cod were reared at constant temperatures from 6 to 20(C and the sagittal otoliths were analysed for oxygen isotope values. We determined that temperature-dependent fractionation of oxygen isotopes in the otoliths was close to that reported for inorganic aragonite at low temperatures, but there were deviations from oxygen isotope fractionation equations for otoliths of other species. The linear relationship between oxygen isotope value in the cod otoliths and temperature was determined to be: 1000 Lna ¼ 16:75ð10 3 TK � 1 Þ� 27:09. Temperature estimates with 1(C precision at the 95% probability level require a sample size of R5 otoliths. Only an insignificant amount of the variance in the data was due to variance between left and right otolith, and due to repeated measurements of otolith subsamples. This study confirms that stable isotope values of cod otoliths can give precise and accurate estimates of the ambient temperature experienced by fish.

Survival of reared and wild Atlantic salmon smolts: size matters more in bad years

Abstract: We compared the marine survival of Carlin-tagged wild and hatchery-reared Atlantic salmon smolts of the Simojoki river, northern Baltic Sea. All the reared and released smolts were the offspring of native spawners returning to the river. Reared smolts were adipose-fin-clipped and released from the hatchery several weeks before tagging. The wild and reared smolts were simultaneously caught and tagged at a smolt trap located at the Simojoki river mouth. The study was conducted in two years, 1991 and 1993, when post-smolt survival in the Baltic Sea was different. Tags were returned by fishermen and return rates were used to estimate the survival of the smolt groups. We applied generalized linear models with survival as response variable and the year, origin, and smolt size as explanatory variables. On average, wild smolts had a 4.5 times higher survival rate than reared fish of the same smolt size. The difference in observed tag recovery rates as such was only about twofold or less, as the larger size of the reared smolts compared with the wild ones compensated for their lower survival rate. The better survival of wild than reared smolts was more pronounced in the low-survival year (1993 smolt year class) than in the high-survival year (1991 smolt year class).

Possible solutions to some challenges facing fisheries scientists and managers

Abstract: The purpose of this paper is to review recent work on four key challenges in fisheries science and management: (1) dealing with pervasive uncertainties and risks; (2) estimating probabilities for uncertain quantities; (3) evaluating performance of proposed management actions; and (4) communicating technical issues. These challenges are exacerbated in fisheries that harvest multiple stocks, and various methods provide partial solutions to them: (i) risk assessments and decision analyses take uncertainties into account by permitting several alternative hypotheses to be considered at once. (ii) Hierarchical models applied to multi-stock data sets can improve estimates of probability distributions for model parameters compared with those derived through single-stock analyses. (iii) Operating models of complete fishery systems provide comprehensive platforms for testing management procedures. (iv) Finally, results from research in such other disciplines as cognitive psychology can facilitate better communication about uncertainties and risks among scientists, managers, and stakeholders.

Modelling fishing location choice within mixed fisheries: English North Sea beam trawlers in 2000 and 2001

Abstract: Numerous studies have proposed methodologies to model fisher behaviour with the aim of predicting the outcomes of decision-making on board a fishing vessel. Both short- and longterm processes (e.g. investment) impact fleet dynamics. The proposed structure of the models has tended to depend upon the nature of the fishery and the control variables (technical restrictions, quotas, effort control, and/or closed areas). For example, within the context of multi-stock, multi-fleet fisheries (mixed fisheries), a skipper will allocate effort (as input to the production process) to harvest a range of species. Spatial complexity is normally excluded in models of behaviour. In this paper, two spatial analyses are presented for modelling location choice: an analysis based on a random utility model (RUM), and a simplified simulation model of individual vessels that depends on the results of the RUM. These models are applied to the English beam-trawl fleet operating in the North Sea in 2000. The results from the RUM indicate that the number of trips, the average trip length, and the average effort in each ICES rectangle are significant variables affecting location choice, in addition to catch rate for the previous year (1999), weighted by value. The last result is used as an assumption in a simulation model of fishing effort, i.e. fishers make decisions on spatial location of operation on the basis of past catch rates. The simulation model is used to predict the distribution of the same fleet for one month during the temporary closure in the North Sea in 2001. The predicted values for effort relate well to the fishing patterns observed. Crown Copyright 2004 Published by Elsevier Ltd on behalf of International Council for the Exploration of

Comparative ecology of over-wintering Calanus finmarchicus in the northern North Atlantic, and implications for life-cycle patterns

Abstract: Data from plankton net and Optical Plankton Counter sampling during 12 winter cruises between 1994 and 2002 have been used to derive a multi-annual composite 3-D distribution of the abundance of over-wintering Calanus finmarchicus in a swath across the North Atlantic from Labrador to Norway. Dense concentrations occurred in the Labrador Sea, northern Irminger Basin, northern Iceland Basin, eastern Norwegian Sea, Faroe–Shetland Channel, and in the Norwegian Trench of the North Sea. A model of buoyancy regulation in C. finmarchicus was used to derive the lipid content implied by the in situ temperature and salinity at over-wintering depths, assuming neutral buoyancy. The Faroe–Shetland Channel and eastern Norwegian Sea emerged as having the highest water column-integrated abundances of copepodites, the lowest over-wintering temperature, and the highest implied lipid content. The results are discussed in the context of spatial persistence of populations, seasonal patterns of abundance, and relationships between over-wintering and lipid accumulation in the surface waters.

The relationship between fish condition and the probability of being mature in American plaice (Hippoglossoides platessoides)

Abstract: Fish condition can be related to a population's reproductive potential in a variety of ways. The relationship between condition and the probability of being mature (adult) was examined in three populations of American plaice (Hippoglossoides platessoides). The effect of condition was tested after first removing the known effects of age and length. Neither relative liver condition nor relative body condition significantly affected the probability of male American plaice being mature, but there was a significant positive effect of both condition indices on the probability of being a mature female. However, the additional variation explained by female condition was small when compared with the combined effect of age and length. Condition is related to the age and size at which fish attain adulthood. Variation in fish condition will have a direct impact on the spawning stock biomass of a population through differences in the maturation schedule of cohorts with differing condition.

Ultrasonic tracking reveals multiple behavioural modes of snapper (Pagrus auratus) in a temperate no-take marine reserve

Abstract: Interactions between marine reserve populations and non-reserve populations of exploited fishes have generally been modelled using simplistic assumptions about behaviour. Consequently, there is a recognized need for better information on fish movement behaviour at the appropriate spatial scales to generate more realistic interaction models. Automated ultrasonic tracking of marine fish, applied in this study, offers the potential to continuously track individuals for periods of up to several years within the study area. Snapper (Pagrus auratus) (FL: 33-68 cm) were surgically implanted with individually coded ultrasonic transmitters to monitor their movement within the Cape Rodney to Okakari Point Marine Reserve in northeast New Zealand from October 2001 to September 2002. The range of movement patterns observed includes some fish resident for the entire tracking period, while others were more mobile. Of the latter group, some fish left the array permanently while others returned after up to 83 days of continuous absence. Nearly all fish showed some level of site fidelity for varying periods of the time they were tracked. Fish activity peaked in the summer, when highest densities are known to occur. Results suggest that snapper behaviour is variable, and that patterns of habitat utilization vary between fish and also seasonally. Such a complex range of behaviours may be a key component for achieving desirable outcomes for both conservation and fisheries in marine reserves.

Small-scale distribution of juvenile gadoids in shallow inshore waters; what role does maerl play?

Abstract: The indirect effects of demersal fisheries, such as habitat degradation, are currently thought to be impacting gadoid stocks. Maerl fulfils nursery area prerequisites for several invertebrate species, so its role in similar ecosystem service provision for gadoids has been addressed. Juvenile cod (Gadus morhua), saithe (Pollachius virens), and pollack (Pollachius pollachius) in shallow (<7 m) inshore waters were surveyed with fykenets and scuba off western Scotland over a period of 12 months. Juvenile densities were highest from September to November, and at that time, significantly more were present during the day and associated with maerl (that lacked macroalgal cover) than with heavily vegetated rocky and gravel substrata. Juvenile cod were present throughout the year, whereas saithe appeared in July, and pollack from September to January. With its abundance of food, maerl probably has a high holding capacity for juvenile gadoids, and thus is an important part of the inshore nursery system.

Do visual transects provide true population density estimates for deepwater fish

Abstract: This study assesses the impact species ecology, fish reactions, and natural behaviour have on visual strip transect counts of deepwater fish carried out with an ROV (remotely operated vehicle). Two terraces and one canyon were visited on the continental slope of the Bay of Biscay. Species such as rabbit fish (Chimaeridae) and North Atlantic codling (Lepidion eques) appear to have avoided the ROV. The vertical distance off the bottom provided evidence that some individuals, in particular slickheads (Alepocephalidae) might have been missed by being above the ROV. GLM modelling showed the importance of depth, current speed, and relative surveying direction on transect counts. Natural and reaction behaviour of deep-sea fish will lead to variable and biased population density estimates.

Identifying global synchronies in marine zooplankton populations: issues and opportunities

Abstract: Analyses of the influences of climate variability on local zooplankton populations and those within ocean basins are relatively recent (past 5-10 years). What is lacking are comparisons of zooplankton population variability among the world's oceans, in contrast to such global comparisons of fish populations. This article examines the key questions, capabilities, and impediments for global comparisons of zooplankton populations using long-term (>10 year) data sets. The key question is whether global synchronies in zooplankton populations exist. If yes, then (i) to what extent are they driven by “bottom-up” (productivity) or “top-down” (predation) forcing; (ii) are they initiated by persistent forcing or by episodic events whose effects propagate through the system with different time-lags; and (iii) what proportion of the biological variance is caused directly by physical forcing and what proportion might be caused by non-linear instabilities in the biological dynamics (e.g. through trophodynamic links)? The capabilities are improving quickly that will enable global comparisons of zooplankton populations. Several long-term sampling programmes and data sets exist in many ocean basins, and the data are becoming more available. In addition, there has been a major philosophical change recently that now recognizes the value of continuing long-term zooplankton observation programmes. Understanding of life-history characteristics and the ecosystem roles of zooplankton are also improving. A first and critical step in exploring possible synchrony among zooplankton from geographically diverse regions is to recognize the limitations of the various data sets. There exist several impediments that must be surmounted before global comparisons of zooplankton populations can be realized. Methodological issues concerned with the diverse spatial and temporal scales of “monitored” planktonic populations are one example. Other problems include data access issues, structural constraints regarding funding of international comparisons, and lack of understanding by decision-makers of the value of zooplankton as indicators of ecosystem change. We provide recommendations for alleviating some of these impediments, and suggest a need for an easily understood example of global synchrony in zooplankton populations and the relation of those signals to large-scale climate drivers.

Water-mass formation and distribution in the Nordic Seas during the 1990s

Abstract: Hydrographic, oxygen and nutrient data collected in the Nordic Seas during the 1990s are presented. During the decade, deep waters originating from the Arctic Ocean, identified by salinities in excess of 34.9, spread into the Greenland Basin. In 1991, these waters extended westward from the mid-ocean ridge to about 2(E. This process continued over time and by 1993 there was a layer with salinities above 34.9 along the entire section, between 7.6(W and the Barents Sea Slope, and probably across the whole basin. In 2000 the basin had these high salinities at depths greater than 1400 m. At 1500 m in the central basin the salinity increase during the decade was 0.012 units, decreasing to 0.006 at 3000 m, and associated temperatures increased by 0.28 and 0.09(C, respectively. This warming more than compensated for the salinity increase so that the density of the deep water decreased during the decade, s3 decreasing by 0.027 kg m � 3 at 1500 m and by 0.006 kg m � 3 at 3000 m. Decreasing oxygen content and increasing concentrations of silicate further indicated the increasing influence of Arctic Ocean Deep Water. Interaction with the atmosphere is decisive for the conditions in the area. In the central Greenland Sea there is close correlation between wind forcing and upper-layer salinity. Significant deep-water formation occurs only during cold winters, or rather, in periods with several succeeding cold winters and the 1960s were the first period in which these conditions occurred since 1920. This is shown by meteorological observations at Jan Mayen since 1921, and at Stykkisholmur, Iceland, since 1823. Relatively high salinities were observed near the bottom over the Iceland Plateau. These waters seem to be derived from Arctic Ocean deep waters that have been diverted from the East Greenland Current, into the East Icelandic Current. While flowing through the Iceland Sea their nutrient concentration increases considerably. This water flows into the Norwegian Basin where it forms a slight salinity maximum around 1500 m, which is associated with a minimum in oxygen content. At greater depths the water masses are from the Greenland Sea. The salinity decreases and the oxygen increases toward approximately 2500 m, from where the trends are reversed toward a slight salinity maximum around 3000 m, where there also is a minimum in oxygen as well as in CFC-11. These characteristics seem to derive from Arctic Ocean Deep Water, floating above waters more characterized by Greenland Sea Bottom water nearest to the bottom as suggested by decreasing salinity and an increase in both oxygen and CFC-11 concentration. This shows that even the very homogeneous Norwegian Sea Deep Water is stratified. There are also slight differences between the deep waters of the basins in the Norwegian Sea. In the Norwegian Basin the deep water has slightly higher salinity, lower dissolved oxygen and higher silicates than the deep water in the Lofoten Basin, and even more so compared with the area west of Bear Island. This shows that the Lofoten Basin and the northern Norwegian Sea are more directly influenced by waters from the Greenland Sea than the Norwegian Basin.

Depensation, probability of fertilization, and the mating system of Atlantic cod (Gadus morhua L.)

Abstract: Rowe, S., Hutchings, J. A., Bekkevold, D., and Rakitin, A. 2004. Depensation, probability of fertilization, and the mating system of Atlantic cod (Gadus morhua L.). e ICES Journal of Marine Science, 61: 1144e1150. Atlantic cod (Gadus morhua L.) have been severely overexploited and are currently at historic population lows, having declined 90% in the North Sea and 99% off northeast Newfoundland in recent decades. Slow rates of recovery and continuing declines may be attributable to depensation, defined as a reduction in per capita growth rate concomitant with reduced population size. Several potential causes of depensation relate to low mating success and consequent reduced production of offspring. We explore the empirical basis of one of these in Atlantic cod using egg fertilization and male abundance data obtained from 21 experimental populations generated by three independent research programmes. We find support for the hypotheses that (a) fertilization rate declines with abundance and (b) variance in fertilization rate increases as population size declines. The former identifies one potential mechanism underlying depensation in Atlantic cod. The latter has negative genetic consequences for effective population size (Ne), resulting in a decline in the ratio of Ne to census population size (Ne/Nc) with declining abundance. Our results may have general implications for the conservation biology of broadcast-spawning marine fish, particularly those with mating systems similar to that of Atlantic cod.

At-sea distribution of female southern elephant seals relative to variation in ocean surface properties

Abstract: We examined the relationships between physical oceanography (sea surface temperature SST, sea surface height anomaly - SSH, ocean colour - OC, bathymetry - BA, sea-ice concentration - SI, and their associated gradients) and the foraging distribution (time at sea) of female southern elephant seals using generalized linear and generalized additive models (GLM and GAM). Using data from 28 separate foraging trips (22 unique individuals) over two years, we found that during the post-lactation trips (summer), the best GLM demonstrated a negative relationship between time at sea and SST and BA, but a positive relationship with SST gradient and SSH. During the post-moult (winter) trips, there was a negative relationship with OC gradient, SSH, and BA. The best post-lactation GAM identified a positive relationship with OC gradient, negative relationships with CC and SST gradient, and a non-linear relationship with SST. For the post-moult trip there was a negative relationship with OC, SST, BA and BA gradient, and a positive relationship with SST gradient. The relationship between the predicted time and observed time at sea was significant only for the post-lactation GAM, although predictability was low. That SST and its gradient predicted a small, but significant proportion of the variation in time at sea is indicative of the frontal zones within this area that are generally more biologically productive than surrounding regions. It appears that coarse-scale oceanographic configuration influences foraging behaviour in southern elephant seals only subtly. Nonetheless, some of the mechanisms influencing predator foraging are congruent with expectations of distribution of marine food resources at coarse spatial scales. (C) 2004 International Council for the Exploration of the Sea. Published by Elsevier Ltd. All rights reserved.

The diet of sperm whales ( Physeter macrocephalus ) in southern Australian waters

Abstract: Stomach contents were collected from 36 sperm whales (Physeter macrocephalus) involved in two mass stranding events during February 1998 along the west coastline of Tasmania, Australia. Contents were dominated by oceanic cephalopods, with a total of 101 883 cephalopod beaks representing 48 species from 14 families of Teuthids, two species from two families of Octopods, and a single Vampyromorph species identified. Species diversity was higher in these animals than other sperm whales studied in the southern hemisphere, with samples containing an average of (±s.d.) 28.4 ± 11.1 species per sample. Diet samples were dominated by subtropical and muscular cephalopod species. Members of the family Histioteuthidae were the most important numerically, and were also important in terms of estimated reconstituted mass, although members of the Onychoteuthidae were the most dominant species in samples in terms of estimated reconstituted mass. Other families numerically important to species composition included the cranchiid, lepidoteuthid, onychoteuthid, and pholidoteuthid families, while the architeuthid, pholidoteuthid, and ommastrephid families were also important in terms of reconstituted mass. Cephalopod species composition varied with stranding site and with sex, but not with age. However, differences did not represent systemic variation with groups marked by high individual variability. Lower rostral lengths of all cephalopod species ranged from 1.3 to 40.7 mm. Calculated dorsal mantle lengths from all species ranged from 10.7 to 2640.7 mm (mean ± s.d. = 233.7 ± 215.7 mm) and estimated wet weights of cephalopod prey ranged from 2.7 to 110 233.1 g (mean ± s.d. = 828.3 ± 3073.6 g). While there were differences in the size of some cephalopod species between stranding sites and with age, this was marked by high individual variability. Differences in diet composition and prey size between sperm whales reflect individual variability in foraging success and perhaps also foraging groups related to the social structure of this species. © 2004 International Council for the Exploration of the Sea. Published by Elsevier Ltd. All rights reserved.

A comparative study of Calanus finmarchicus mortality patterns at five localities in the North Atlantic

Abstract: Author(s): Ohman, MD; Eiane, K; Durbin, EG; Runge, JA; Hirche, HJ | Abstract: We compare the patterns of stage-specific mortality of Calanus finmarchicus at five localities across the North Atlantic Ocean during the spring-summer period of active population growth: Georges Bank, a continental shelf locality in the NW Atlantic, based on 30 broadscale survey cruises in the US GLOBEC program; the northern North Sea, studied during the historic FLEX program with sampling four times daily for 73 days; Ocean Station M in the central Norwegian Sea, based on an 80-day daily time-series; and Lurefjorden (sampled weekly in late winter-early summer) and Sorfjorden (sampled monthly), two fjords in southwestern Norway characterized by markedly different guilds of predators. The mortality estimation methods included Wood's Population Surface Method, the Vertical Life Table (VLT) method, and a modified VLT, according to the study site and copepod recruitment schedules. Contrary to assumptions implicit in many simulation models and indirect methods for estimating zooplankton mortality, both rates and stage-specific patterns of mortality of C. finmarchicus vary appreciably across the North Atlantic. Characteristics of local environments, including the predator field in particular, appear to strongly influence mortality schedules in different regions. In at least two sites (Georges Bank and Ocean Station M), mortality rates of early stages of C. finmarchicus are density-dependent. We attribute this density-dependent mortality to egg cannibalism, which introduces non-linear population responses to changing environmental conditions. Region-specific biological interactions can substantially modify the effects of physical climate variability and render simple linear relationships between climate and zooplankton abundance unlikely. © 2004 International Council for the Exploration of the Sea. Published by Elsevier Ltd. All rights reserved.

Diel patterns in pelagic fish behaviour and distribution observed from a stationary, bottom-mounted, and upward-facing transducer

Abstract: Diel patterns in pelagic fish behaviour and distribution observed from a stationary, bottom- mounted and upward-facing transducer

Developing and refining a joint management procedure for the multispecies South African pelagic fishery

Abstract: Pilchard (sardine) and anchovy are the main targets of South Africa’s pelagic fishery. This fishery is the country’s second most valuable in monetary terms, and produces the highest annual yield in terms of landed mass (in recent years, a combined catch of the order of 400 000 t). It is the most dynamic of South Africa’s main commercial fisheries, because the species targeted are relatively short-lived, often occur in mixed shoals, and experience large fluctuations in abundance. Mixed shoaling causes operational problems for the fishery, because of the inevitability of juvenile pilchard bycatch (of more value as adults for canning) in the anchovy-directed fishery. This operational interaction implies a trade-off between allowable catches for the two species, and hence necessitates that they are managed together. The development of a joint ‘‘management procedure’’ (sensu IWC) for the two species is described. This provides a framework for quantifying this trade-off, subject to the constraint that acceptable levels of risk of ‘‘collapse’’ are not exceeded for either resource. Important new features incorporated in a revision of the procedure implemented in 2002, which have made appreciably enhanced catches from the resources possible, are described.

Three-dimensional structure and avoidance behaviour of anchovy and common sardine schools in central southern Chile

Abstract: We studied the avoidance behaviour and three-dimensional (3-D) structure of anchovy (Engraulis ringens) and common sardine (Strangomera bentincki) schools mixed in high concentrations in a coastal area of central southern Chile. Observations were carried out during an acoustic survey in January 2002 by means of a vertical echosounder and a multi-beam sonar. The sonar harvested around 900 series of 3-D school images, and 3000 2-D school images were collected with the echosounder. The results showed that all fish aggregations presented the same internal structure, but different global morphologies, from single small schools (with length three times the height) on the edges of the distribution to large dense layers (length more than seven times the height) in its centre. Observation of avoidance in the vertical and horizontal planes indicated that limited vertical diving occurred close to the ship (fish dive from the surface to the 5-10-m depth layer below the vessel), while no horizontal avoidance was observed.

Dynamics of fisheries, and the flexibility of vessel activity in Denmark between 1989 and 2001

Abstract: Ulrich, C., and Andersen, B. S. 2004. Dynamics of fisheries, and the flexibility of vesselactivity in Denmark between 1989 and 2001. e ICES Journal of Marine Science, 61:308e322.Danish fishing vessels can be characterized by their diversity of fishing practice in terms offishing gear and target species, and by their operational flexibility in respect of these fishingpractices throughout the year. We describe the temporal fluctuations in this flexibility byfollowing the activity of individual fishing vessels between 1989 and 2001. Initially,a typology of fisheries (classification of fishing trips) and vessel groups (classification offishing vessels) was established through multivariate analyses of catch and effort data for1999. In all, 54 fisheries and 25 vessel groups were identified. These typologies were thenapplied to all data for the whole time period, and the dynamics of fisheries and vesselgroups investigated. The dynamics of vessels groups are studied both within groups (mainand secondary fisheries, changes in activity patterns) and between groups (tracking ofvessels shifting between groups). Results show average stability of vessel activity in termsof the main fishery, along with a great diversity of secondary fisheries and somepossibilities for shifting between gears and areas. We conclude that the level of technicalinteractions is high, and that separation into distinct management units is difficult.

From single-species advice to mixed-species management: taking the next step

Abstract: Vinther, M., Reeves, S. A., and Patterson, K. R. 2004. From single-species advice to mixed-speciesmanagement:takingthenextstep.ICESJournalof MarineScience,61:1398e1409.Fishery management advice has traditionally been given on a stock-by-stock basis. Recentproblems in implementing this advice, particularly for the demersal fisheries of the NorthSea, have highlighted the limitations of the approach. In the long term, it would be desirableto give advice that accounts for mixed-fishery effects, but in the short term there is a needfor approaches to resolve the conflicting management advice for different species within thesame fishery, and to generate catch or effort advice that accounts for the mixed-speciesnature of the fishery. This paper documents a recent approach used to address theseproblems. The approach takes the single-species advice for each species in the fishery asa starting point, then attempts to resolve it into consistent catch or effort advice using fleet-disaggregated catch forecasts in combination with explicitly stated management prioritiesfor each stock. Results are presented for the groundfish fisheries of the North Sea, and theseshow that the development of such approaches will also require development of the ways inwhich catch data are collected and compiled.

Swimming endurance of haddock ( Melanogrammus aeglefinus L.) at prolonged and sustained swimming speeds, and its role in their capture by towed fishing gears

Abstract: This paper describes an experiment to determine the swimming endurance of haddock (Melanogrammus aeglefinus) at prolonged swimming speeds. Fish were stimulated to swim in a circular path around an annular tank, using a moving light pattern to trigger the optomotor response. Individually tagged haddock (length range 16.0e40.2 cm) swam in groups over a range of speeds (0.3e0.9 m s � 1 ) and at a constant temperature (9.85G 0.07(C). Endurance of individual fish was shown to be related to their swimming speed and length. However, there was also significant variation (p! 0.05) in the performance of fish of approximately equal length. Distinct behaviours and swimming gaits were also identified and associated with the performance of individual fish. The inverselinear model is introduced, as an alternative to the log-linear model, for describing the relationship between swimming speed and endurance, and estimating maximum sustainable swimming speed (Ums). Estimates of Ums ranged from 0.38G 0.03 m s � 1 and 3.16G 0.02 BL s � 1 (for a 16.0-cm fish) to 0.62G 0.04 m s � 1 and 1.51G 0.07 BL s � 1 (for a 42.0-cm fish). Ums represents an important threshold in the behavioural physiology of fish, marking the upper limit of aerobic swimming. The relevance of these results and Ums to the fish capture process is discussed.

A multidisciplinary study of the immediate effects of mechanical clam-harvesting in the Venice Lagoon

Abstract: Pranovi F., Da Ponte F., Raicevich S., and Giovanardi O., 2004. A multidisciplinary study of the immediate effects of mechanical clam harvesting in the Venice Lagoon. e ICES Journal of Marine Science 61: 43e52. In the Venice Lagoon, clam (Tapes philippinarum) harvesting is carried out mainly by means of a gear locally called ‘‘rusca’’, developed by local fishermen. The rusca consists of an iron cage, an outboard engine propeller, which produces a water flow directed onto the bottom suspending sediments and fauna, and a net bag where the clams are collected. The aim of this study was to provide an analysis of the immediate effects of rusca fishing on a wide spectrum of ecosystem compartments: sediment biogeochemistry, sediment resuspension, and macro- and meiofauna community. Rusca fishing produced a V-shaped furrow (about 60 cm wide and 7 cm deep) and a plume of resuspended sediment with a significant increase (up to two orders of magnitude greater than undisturbed areas) of suspended particulate matter (SPM) and increased Ctot ,C org ,N tot, and sulphide concentrations in the water column. Experimental rusca hauls significantly reduced macrofauna density, while no significant effect on meiofauna was detected. Results are also discussed in terms of basin-scale impact, attempting to compare natural and anthropogenic disturbance.

Diel vertical distribution of fish larvae during the winter-mixing period in the Northwestern Mediterranean

Abstract: The vertical distributions of the larvae of shelf and oceanic fish species that spawn during the winter-mixing period in the Mediterranean are described from 22 vertically stratified plankton tows. Diel differences in the vertical distribution patterns in relation to physical data and potential prey abundance throughout the water column were examined. Even in absence of stratification, the larvae of the various fish species showed different patterns of vertical distribution and diel changes. The larvae of shelf-dwelling species were found in the surface layers, mainly above 50-m depth, and with some exceptions, with very little diel variation in depth distribution. Therefore, the vertical distribution of the larvae of these species coincided with the maximum concentrations of their potential food, nauplii and copepodite stages of copepods. The larvae of mesopelagic fishes showed deeper distributions in the water column and most of these species were located closer to the surface during the day than at night. Given the homogeneity of the physical characteristics throughout the water column, except for light, this behaviour may be determined not only by the higher concentration of prey in the surface layers but also by adequate light levels for feeding.

Response of clupeid fish to ultrasound: a review

Abstract: A number of species of clupeid fish, including blueback herring, American shad, and gulf menhaden, can detect and respond to ultrasonic sounds up to at least 180 kHz, whereas other clupeids, including bay anchovies and Spanish sardines, do not appear to detect sounds above about 4 kHz. Although the location for ultrasound detection has not been proven conclusively, there is a growing body of physiological, developmental, and anatomical evidence suggesting that one end organ of the inner ear, the utricle, is likely to be the detector. The utricle is a region of the inner ear that is very similar in all vertebrates studied to date, except for clupeid fish, where it is highly specialized. Behavioural studies of the responses of American shad to ultrasound demonstrate that they show a graded series of responses depending on the sound level and, to a lesser degree, on the frequency of the stimulus. Low-intensity stimuli elicit a non-directional movement of the fish, whereas somewhat higher sound levels elicit a directional movement away from the sound source. Still higher level sounds produce a “wild” chaotic movement of the fish. These responses do not occur until shad have developed the adult utricle that has a three-part sensory epithelium. We speculate that the response of the American shad (and, presumably, other clupeids that can detect ultrasound) to ultrasound evolved to help these species detect and avoid a major predator - echolocating cetaceans. As dolphins echolocate, the fish are able to hear the sound at over 100 m. If the dolphins detect the fish and come closer, the nature of the behavioural response of the fish changes in order to exploit different avoidance strategies and lower the chance of being eaten by the predators.