Robin Cook

Predator ingestion rate and its bearing on feeding time and the theory of oprimal diets

SUMMARY (1) Two insect predators, Adalia bipunctata (L.) and Notonectaglauca (L.) are shown to have ingestion rates which decline over the time whilst the animal is feeding on a single prey item. The decline is not due to satiation. (2) A model which maximizes the average rate of energy harvest is applied to the ingestion curve effect. The length of time spent feeding on a prey item is determined by this average rate and predicts that feeding times will be shorter in more profitable environments. (3) Experiments support the prediction that feeding time per item and the amount of food eaten per item decreases as the intercatch interval shortens. Unlike predictions from a gut limiting hypothesis, feeding times seem to be related to the average intercatch interval not the individual intercatch intervals. (4) The ingestion rate effect is incorporated into an optimal diet model. Prey types should not be ranked by their average profitability but by their unexploited profitability. (5) The modified optimal diet model will predict a variety of functional response curves dependent on the form of the ingestion curve. These include double plateau and sigmoid curves. The model offers a means of studying multi-prey type functional responses from an optimal foraging point of view.

OPTIMAL FORAGING BY PARASITOID WASPS

(1) Experiments have been described in which insect parasites were observed continuously searching in a patchy host habitat. (2) The results of these experiments have been used to examine in greater detail the predictions of the Cook & Hubbard (1977) model for the outcome of search by insect parasites and predators. (3) The observed final rates of encounter between parasites and healthy hosts, the final density of healthy hosts in each patch at the end of the interaction, the giving up times for each patch, the effect of exploitation on the time budget, and the effect of the range of patch types available to the parasites all show an encouraging measure of agreement with the predictions of the model. (4) The present view of parasite mutual interference, deviations from the expected relationships and the important role of sampling strategies have all been discussed in the light of the results obtained in this study.

Reversal of fish stock decline in the Northeast Atlantic

Analyses of global fish stocks paint a mixed picture of success, with some holding fishery management responsible for the poor status of many stocks [1-3] or predicting widespread collapse [1, 4]. Some suggest a stable [5] or improving situation [6] in certain jurisdictions. The debate is particularly polarized in the European Union, where the Common Fisheries Policy (CFP) has been criticized for failing to protect stocks [2, 7-10], while others argue that a rebuilding process is underway [11, 12]. We show that substantial change in stock trends occurred in the area around the turn of the century: since then, the fishing pressure (as measured by the exploitation rate) has reduced continuously and there have been increases in biomass, demonstrating the potential for stock recovery. In 2011, for the first time, the majority of assessed stocks, where reference points are defined, were fished sustainably. The reductions in fishing pressure were associated with declines in fishing effort. The last reform of the CFP, in 2002, introduced effort control as part of more enforceable management measures, which were also based on longer-term plans. Further reforms to the CFP are currently being developed, so it is important, when correcting its weaknesses, to also acknowledge and build on the success of a major reduction in the fishing pressure on European fish stocks.

Apostatic selection as an optimal foraging strategy.

Two simple optimal diet models are shown to be capable of predicting a variety of effects similar to those of frequency-dependent selection, the type depending on whether the predator can search simultaneously for the prey species it attacks, or not. Intensity of detected selection should decline with increasing prey exploitation.-from AuthorsDept. of Biological Sciences, The Univ., Dundee DD1 4HN, & The Marine Lab., Victoria Rd., Aberdeen, Scotland, UK.

The magnitude and impact of by-catch mortality by fishing gear

This chapter discusses the magnitude and impact of by-catch mortality by fishing gear. It is part of a collection which addresses ecological and environmental issues associated with responsible and sustainable marine fisheries.

Ecosystem Approaches for Fisheries Management

Ecosystem Approaches for Fisheries Management: University of Alaska Sea Grant, AK-SG-99-01, Fairbanks, AK, 1999, 756 pages, hardback, ISBN 1-56612-061-6 (

Cascading ecological effects of eliminating fishery discards

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Fish bycatch and discarding in Nephrops trawlers in the Firth of Clyde (west of Scotland)

Discarding by fisheries is perceived as contrary to responsible harvesting. Legislation seeking to end the practice is being introduced in many jurisdictions. However, discarded fish are food for a range of scavenging species; so, ending discarding may have ecological consequences. Here we investigate the sensitivity of ecological effects to discarding policies using an ecosystem model of the North Sea—a region where 30–40% of trawled fish catch is currently discarded. We show that landing the entire catch while fishing as usual has conservation penalties for seabirds, marine mammals and seabed fauna, and no benefit to fish stocks. However, combining landing obligations with changes in fishing practices to limit the capture of unwanted fish results in trophic cascades that can benefit birds, mammals and most fish stocks. Our results highlight the importance of considering the broader ecosystem consequences of fishery management policy, since species interactions may dissipate or negate intended benefits.

Grey seal predation impairs recovery of an over‐exploited fish stock

Nephrops norvegicus(Norway lobster or prawn) is an important target species in demersal fisheries of the north-east Atlantic. Trawling for Nephrops is wasteful when many small fish are caught and discarded in the process. Here, data from 106 commercial fishing trips, sampled between 1982 and 1998 as part of the Scottish discard sampling programme, are used to investigate the fish bycatch and discards of Nephrops trawlers in the Firth of Clyde (west of Scotland). A large proportion of the fish caught in the Clyde is discarded, the discards mainly consisting of small demersal fish (mean length about 19 cm), particularly young whiting (Merlangius merlangus). Within the study period, annual estimates of total fish discards ranged from 318 to 3 027 tonnes, with a mean of 1 761 tonnes. Fish landings and discards biomass per unit effort both decreased over the study period. However, the decline in landings per unit effort was greater than that in discards per unit effort, corresponding to an increase in the discard rate over time. In recent years, discards have comprised about 70 % of the fish bycatch. The mean length of discarded fish was positively related to mesh size.

Sustainable exploitation of temperate fish stocks

Grey seal predation has been blamed by fishers for the decline of Atlantic cod stocks and has led to calls for seal culls. In the West of Scotland, estimates of cod consumption by seals have exceeded reported catches and spawning biomass, focussing attention on the interaction between fishers and seals. Bayesian models making different assumptions about seal predation were used to estimate the size of the West of Scotland cod stock between 1985 and 2005 and the mortalities due to fishing and seal foraging. A simple population model was used to identify the likely direction of cod population change at recent mortality rates. All model configurations suggest that the total mortality of cod has remained fairly stable and high for many years regardless of the assumptions on seal predation. The high mortality explains the long-term decline of the stock. The best-fitting model suggests that mortality due to fishing reduced substantially in the decade up to 2005, but has been replaced by increased seal predation mortality on a smaller cod stock. Given total mortality estimates, the stock is unlikely to recover even at present reduced levels of fishing. Synthesis and applications. Our model offers a method of estimating seal predation mortality as part of routine stock assessments that inform fishery management. The analysis shows that predation by seals can be an important component of the total stock mortality. It also shows that assuming invariant natural mortality, as adopted in many standard fish stock assessments, may lead to incorrect perceptions of fishing mortality, over-estimating the benefits of reducing fishing mortality when there is density-dependent predation. It is essential to consider predation by top predators when formulating appropriate advice for managing the fishery.