Publisher Summary This chapter discusses the field investigation of the early life stages of marine fish. The motivations for investigations have changed little over the intervening period, being mainly the assessment of adult spawning biomass and distribution, and the desire to understand how environmental variations and changes in the abundance of other species interact to regulate the abundance in particular fish populations. In many cases, the annual recruitment of new juveniles into the population is the most important process governing population fluctuations. The factors affecting recruitment, particularly those affecting the survival of the eggs and larvae, are therefore perceived to be of key importance. In the review of the chapter, the progress of field research on the early life stages of marine fin fish is examined. The objectives of field studies are briefly stated in the chapter, followed by a review of sampling methodology, analytical techniques, and the methods for assessing adult spawning biomass from egg and larval abundance data. This chapter also discusses the field evidence for variability in the growth, survival, and behavior in relation to biological and physical oceanographic conditions concentrating on larvae but including eggs were relevant. Finally, the match-mismatch and member-vagrant hypotheses, and the role of density-dependence in early life stage regulation, are considered in the light of the field evidence.

Field Investigations of the Early Life Stages of Marine Fish

Status and trends of the nation's biological resources

A growlng body of evidence suggests that surv~val of the planktonic early life stages of marine fish varies spatially, often in concordance with mesoscale and larger circulation patterns and with spatlal variation in biotic and abiotic factors. Environmental variation experienced by individuals may contribute to variance within the population. Detailed spatial information about life histories cannot be obtained however, by simply subjecting individuals to identical time series of environmental variates, or adding random noise to the mean values of those variates. In this paper, w e present a numerical biophysical model to address this problem. The model combines a 3-dimensional hydrodynamic model with a probabilistic, life-stage model of young flsh. The oceanographic model is an eddyresolving, semi-spectral primitive equa t~on model (SPEM) adapted for Shelikof Strait and the western Gulf of Alaska, USA. The biological component is an individual-based model (IBM) which follows each fish through its first 3 mo of life. The 2 models are coupled via a float-tracking algorithm, which advects each fish through the model grid based on the time-varying velocity field derived from the SPEM model, and returns information to the IBM on any physical variables of interest (e.g. temperature and salinity) at the individual's new location. These variables are used to drive biological processes in the IBM. Simulation 1 demonstrates how a probabilistic 1BM using Lagrangian temperature information derived from float tracking can yield significantly different b~ological results than the same model with constant temperature, or a model where all individuals are subjected to identically increasing tlme series of temperature. In Simulation 2 w e compare the float tracks and growth of individuals using a scenario of constant depth for each life stage versus algorithms that include mechanisms producing variable depth. These mechanisms differ for each life stage (e.g. changes in depth due to development of eggs, or die1 migration of feeding larvae). This simulation shows that inclusion of specific mechanisms which determine depth of individual eggs or larvae at particular periods of their development are Important in determ~ning the direction of horizontal advect~on, and the history of exposure to environmental variables for each individual. Simulation 3 compares modeled distributions of the early l ~ f e stages with distributions derived from field surveys.

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Development of a spatially explicit, individual-based model of marine fish early life history

The cntical stage in the iife history of waiieye poiiock Theragra chalcogramma at which recruitment is largely deterrnined has shifted in the Guif of Alaska population. This change foilows a major environmental regime-shift in the late 1970s and subsequent dorninance of the ecosystem by long-lived predatory flatfishes and cod. An exploratory iife table of data on stage-specific abundances was constructed to analyze decadal-scale changes in population demographics. Prior to the mid-1980s, recruitment was correlated with larval mortality, which was largely infiuenced by environmental con- ditions. After the mid-1980s, the larval mortalitylrecruitment relationship eroded, and there was a trend for greater,juvenile mortality coincidmg with an increase in the abundance of predatory flatfishes and cod. Control of recruitment appears to have shifted in recent years from environmental effects on larvae to biological control of juveniles. Ecosystem change and regirne status need to be considered when evaluating mechanisms underlying fish-population dynamics.

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Shifting control of recruitment of walleye pollock Theragra chalcogramma after a major climatic and ecosystem change

The evolutionary importance of introgressive hybridization has long been recognized by plant evolutionists, and there is now a growing recognition for its potential role in animals as well. Detailed empirical investigations of this evolutionary process, however, are still lacking in many animal groups, particularly in the marine environment. Using integrated microsatellite DNA data (eight loci analysed over 803 individuals representing 17 sampling locations) and multivariate statistical procedures (principal component, factorial correspondence and admixture proportion analyses), we: (i) provide a detailed dissection of the dynamics of introgressive hybridization between Sebastes fasciatus and S. mentella, two economically important redfishes from the North-west Atlantic; and (ii) infer the factors potentially involved in the maintenance of the hybrid zone observed in the gulf of St. Lawrence and south of Newfoundland. This study provided one of the rare examples of extensive introgressive hybridization in the ocean, and highlighted the predominant role of this process in shaping the extent of genetic diversity, interspecific differences and population structuring among redfishes from the North-west Atlantic. The extensive (average rate of introgression = 15%) but geographically circumscribed and asymmetrical pattern of introgressive hybridization, the sympatric persistence of two reproductively isolated introgressed groups, the differential patterns of linkage disequilibrium among samples, and the maintenance of genetic integrity of both species outside the defined zone of introgression despite high potential for gene flow, all implicated selection in promoting and maintaining the observed pattern of introgression.

Evidence for broadscale introgressive hybridization between two redfish (genus Sebastes) in the North-west Atlantic: a rare marine example

Current information on systematics and morphology of larvae and juveniles ofSebastes is reviewed, as are intrageneric relationships among the many species of the genus. Although some subgenera may remain after a cladistic analysis of the genus, others are probably artificial groupings of species that have convergent morphologies because of similar ecology. The relationships ofSebastes to other scorpaeniform fishes as well as the relationships of scorpaeniforms to other fishes are not resolved. Of the 102 species ofSebastes worldwide, 69 species have been illustrated as preflexion larvae, 35 as postflexion larvae and 65 as pelagic juveniles. Morphological characters of each of these stages were used to compare species, group them by similar appearances and then compare these groupings with existing subgenera. Some pigment patterns of preflexion larvae were rather consistent among species within certain subgenera but quite variable among species in other subgenera. Postflexion larvae fell into 11 groups based on pigment, head spines and body shape. These groups were not closely aligned with the subgeneric assignments of the component species. On the basis of pigment patterns of pelagic juveniles, six groups were evident, but the specific composition of these groups bore little resemblance to either the larval or subgeneric groups.

Systematics and identification of larvae and juveniles of the genus Sebastes

Walleye pollock, Theragra chalcogramma, form dense aggregations during a brief spawning period from late March to mid-April in Shelikof Strait, Alaska. Spawning produces a large (order 20 × 50 km or more) “patch” of eggs at depth (generally > 150 m), and hatching larvae often produce a “patch” in the upper 50 m. Patches can be defined as coherent features using graded concentration isopleths, and the mean concentration within patches has been observed to be as much as 68 times (for eggs) and nearly 6 times (for larvae) the background concentration. Larval patches drift southwestward and have been identified for about 30 days after hatching in some years. Data are presented for spawner biomass and for early life stages, as available, for 1981, 1983 and 1985. When comparing 1985 with 1981 (2 years with the best coverage), spawner biomass and mean egg concentration within the patch declined concordantly. Larval concentrations about 10 days after hatching differed widely, however: concentrations in early May 1985 were more than an order of magnitude lower than expected. Unlike either 1981 or 1983, no larval patch could be identified in late May 1985; this appears to be attributable to changes detected earlier in the month. The “apparent” mortality rate for a 10-day period after hatching in 1985 was about 0.50 d−1 greater than in 1981. Larval feeding conditions can be excluded as a likely cause of this interannual difference, but predation and advection cannot be. Our findings emphasize the short time period over which significant changes can occur, as well as additional sampling which must be done in future studies. We show that part of the emerging 1985 larval year class could have been removed by cross-channel disturbances in the flow field through Shelikof Strait.

Interactions of a mesoscale patch of larval fish (Theragra chalcogramma) with the Alaska Coastal Current

A major area of research in fisheries oceanography examines relationships between recruitment dynamics of fish populations and the marine environment. A primary goal is to understand the natural causes of variability in year-class strength of commercially valuable species and apply this knowledge to management [Perry, 1994]. The paradigm that the majority of mortality occurs during transport of early life history stages from spawning to nursery grounds [Rothschild, 1986; Houde, 1987] provides an initial temporal focus for most research. The spatial domain includes the region occupied by early life history stages. Since global climate variability impacts regional ecosystem dynamics, however, the spatial domain often must be expanded. The relative importance and manifestation of biological factors [starvation and predation] that limit survival varies each year. Marked interannual and longer period variations in temperature (an influence on metabolic rates and behavior), transport of planktonic stages, and turbulence can exert an influence on both survival of early life history stages, and distribution of juveniles and adults. To understand how these environmental factors influence reproductive success of fish stocks also requires knowledge of the impact of these factors on predators and prey throughout the food web.

An example of fisheries oceanography: Walleye pollock in Alaskan waters

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Arthur W. Kendall

Papers

Systematics and identification of larvae and juveniles of the genus Sebastes

Interactions of a mesoscale patch of larval fish (Theragra chalcogramma) with the Alaska Coastal Current

An example of fisheries oceanography: Walleye pollock in Alaskan waters

11. Biophysical Processes Relevant to Recruitment Dynamics of Walleye Pollock (Theragra chalcogramma) in the Eastern Bering Sea