Showing papers by "Paul J. B. Hart published in 2006"

Intraspecific food competition in fishes

Abstract: Intraspecific food competition exerts powerful selective forces on all animals; successful foragers thrive relative to weaker conspecifics. Understanding competition is therefore fundamental both to ecological insight and to conservation efforts. Fish are adaptable and tractable experimental organisms, offering excellent model systems for studies on competition, and they lend themselves to two approaches: (i) studies of short-term competition, which quantify the components of behavioural interactions; (ii) studies of long-term interactions, in which the indeterminate nature of fish growth makes it possible to measure rates directly and correlate them with competitive success. The nature and the intensity of competition vary according to resource characteristics and distributions in time and space, the ecological context, and the relative competitive abilities of the foragers. Second-order effects, such as winner and loser consequences, add to the complexity and frustrated early attempts to develop realistic models of intraspecific competition. Recently, however, considerable advances have been made in both laboratory and field studies on fishes adding to our understanding of these interacting effects. At the same time, the application of individual-based modelling offers the prospect of progress towards greater realism and accuracy in predicting competitive outcomes. This review draws together a wide and disparate literature on intraspecific competition in fishes to facilitate the work of both empiricists and theoreticians towards these important goals. In the short term, competing individuals may adopt different behavioural strategies and feeding patterns or establish dominance hierarchies and feeding territories. In the longer term, competition can drive character displacement and the formation of species pairs and fish provide some of the most compelling examples of these processes in evolutionary biology. The challenge for the future is to further develop our understanding of the relationship between the competitive environment and the responses of fishes, particularly with closer co-operation between empiricists and theoreticians, and to apply this knowledge to aquaculture and to better management of exploited fish stocks.

Conservation benefits of temperate marine protected areas: variation among fish species.

Abstract: : Marine protected areas, and other fishery management systems that impart partial or total protection from fishing, are increasingly advocated as an essential management tool to ensure the sustainable use of marine resources. Beneficial effects for fish species are well documented for tropical and reef systems, but the effects of marine protected areas remain largely untested in temperate waters. We compared trends in sport-fishing catches of nine fish species in an area influenced by a large (500-km2) towed-fishing-gear restriction zone and in adjacent areas under conventional fishery management controls. Over the period 1973–2002 the mean reported weight of above-average-sized (trophy) fish of species with early age at maturity and limited home range was greatest within the area influenced by the fishing-gear restriction zone. The reported weight of trophy fish of species that mature early also declined less and more slowly over time within the area influenced by the fishing-gear restriction zone. Importantly, the mean reported weight of trophy fish of species that mature late and those that undertake extensive spatial movements declined at the same rate in all areas. Hence these species are likely to require protected areas >500 km2 for effective protection. Our results also indicated that fish species with a localized distribution or high site fidelity may require additional protection from sport fishing to prevent declines in the number or size of fish within the local population.

Subhabitat selection by foraging threespine stickleback ( Gasterosteus aculeatus ): previous experience and social conformity

Abstract: Any mechanism that allows animals to increase their foraging efficiency is likely to be selected for, including the ability to learn to recognise and subsequently discriminate between habitat types based on their profitability. In a series of laboratory studies, we manipulated prey densities across two different experimental subhabitats and demonstrated that threespine stickleback (Gasterosteus aculeatus) can develop foraging preferences for subhabitats that have previously yielded prey. Fish were not recalling the spatial location of prey patches; rather, they were discriminating between subhabitats based on foraging experience there and allocating foraging effort accordingly. Foraging preferences took around 14 days to develop, and once established, they persisted independently of experimental prey density, suggesting that fish were using experience rather than real-time sampling to select foraging grounds. When we presented focal fish with social information cues, we found that they preferentially used local enhancement and current public information cues when they conflicted with previous experience, but that they did not use prior public information. This suggests that in the presence of conspecifics, individuals prioritise social conformity over the use of private information. We discuss our results in the context of optimal foraging and the trade-offs associated with balancing conflicting private and social information.

Quantitative analysis of dental microwear in threespine stickleback: a new approach to analysis of trophic ecology in aquatic vertebrates

Abstract: 1. The threespine stickleback Gasterosteus aculeatus is an important model organism in studies of genomic and phenotypic evolution, adaptation and speciation. Fossil Gasterosteus offer the potential to test models derived from studies of extant fishes over true evolutionary time-scales. Competition for food resources, for example, plays an important part in stickleback speciation, causing divergence in food gathering traits and ecological character displacement, but it is not possible to test this model in fossils because evidence of diet is almost never preserved. 2. We demonstrate here that quantitative analysis of dental microwear, a technique previously applied only to mammals, provides a reliable guide to the dietary preferences of stickleback. Teeth from stickleback raised under laboratory conditions exhibit microwear patterns that vary systematically according to substrate coarseness and whether fishes feed on Daphnia within the water column, or on chironomid larvae from the bottom. Furthermore, microwear data exhibit a progressive shift in their distribution that tracks differences in experimental feeding treatments. 3. Microwear in wild populations also exhibits a relationship with feeding. In blind assessments of trophic niche based on microwear patterns we were able to correctly assign all but one equivocal population to trophic group. Microwear data from wild stickleback exhibit a shift in distribution comparable with that observed across the range of treatments in the laboratory and these allow populations to be ranked according to the degree to which they approach fully benthic or fully limnetic feeding. 4. Our results demonstrate that microwear has the potential to be a powerful tool in the analysis of fish trophic ecology, particularly in the analysis of species pairs and niche differentiation. It has advantages over the trophic snapshot provided by analysis of stomach contents in that microwear reflects feeding and food preferences over a longer period of time, and can be applied where these data are unavailable. Furthermore, it is applicable to extinct organisms and fossils, allowing the role of trophic ecology, niche partitioning and competition over evolutionary time-scales to be investigated for the first time.

Modelling food web interactions, variation in plankton production, and fisheries in the western English Channel ecosystem

Abstract: To explore the contributions that fishing, trophic interactions and plankton production make to explanations of the observed variation of higher trophic (principally fish) levels in the western English Channel ecosystem, Ecosim simulations were run from 1973 to 1999 using the most complete data set yet assembled. The results indicate that a bottom-up mechanism plays an important role in the system production. Inclusion of a primary producer biomass forcing term, estimated from empirical data, improved the goodness of fit of the model estimates to the available biomass data by about 25% compared to fitting using only the series of fishing mortalities. Model fitting was further improved by changing the so-called vulnerability parameters, causing an overall improvement of 62% in explained variation. Incorporating the new vulnerability values, the model was used to estimate a primary production anomaly function to replace the primary producer biomass forcing in driving the model simulations. In this scenario, the model estimated a series of values for primary producer abundance that approximated the empirical data, but gave lower estimates than were observed towards the end of the period. This version also gave a better fitting to the zooplankton abundance data and generally improved the fitting to all functional groups.

Kleptoparasitic prey competition in shoaling fish: effects of familiarity and prey distribution

Abstract: Familiarity is thought to stabilize dominance hierarchies and reduce aggressive interactions within groups of socially living animals. Though familiarity has been widely studied in shoaling fish, few studies have investigated changes in prey competition as a function of time spent together within groups of initially unfamiliar individuals. In this study, we created shoals of threespined stickleback (Gasterosteus aculeatus) and monitored changes in foraging rates and related competitive behaviors within shoals over a 4-week period in experimental series where prey was spatially and temporally concentrated or dispersed. Prey share was unequal under both prey distribution modes, and disparity in prey share was not seen to change as trials progressed. Interestingly, the contest rate for prey items fell over time when individuals were competing for dispersed prey but not when prey were concentrated. We found no evidence that fish showed association preferences for either group members that had consumed a greater or lesser proportion of prey during trials. Though the intensity of competition may be reduced by increased group stability in nature, this is likely to be strongly dependent on the way prey resources are distributed through space and time. roup living is widespread in nature, and animals may actively aggregate for a variety of reasons. Individuals that forage together bear lower per capita vigilance and predation risk costs and stand to gain from potentially higher prey detection rates compared with those foraging alone. A detriment of social foraging is the increased potential for competition with conspecifics for prey once it is discovered because animals are often compelled to compete among themselves in order to maximize their share of it. Competition can be costly in terms of time and energy expenditure or risk of injury or predation, and selection should favor the adoption of behaviors that most efficiently minimize the intensity or duration of conflict while simultaneously preserving the benefits of sociality (Giraldeau and Caraco 2000).

Predicting resource utilization: the utility of optimal foraging models

Abstract: Predicting how predation changes communities is an important challenge for ecologists. One view assumes that predictions can be made in terms of behavioural phenomena. Static optimality methods have been used to devise models of prey choice and are thought to have performed well. Two examples from the literature show that models of diet choice have either been applied without first testing them at the individual level, or have been tested in a way that does not fulfil all their assumptions. A discussion of the nature of mathematical models shows why it is dangerous to translate theories into natural language and how important it is to appreciate the limitations on the mathematical functions describing behaviour. A final section discusses behavioural phenomena such as satiation and learning, which are likely to limit the predictive capacities of static optimization models. It is likely that dynamic models are the way forward although they may be harder to apply at the ecological level.