A model at the level of the foraging trip for the indirect effects of krill (Euphausia superba) fisheries on krill predators

1. Although the development of fisheries for krill in the southern oceans has prompted considerable work on the indirect effects of fisheries on krill predators, to date all work has focused on population level effects. Here, we present for. the first time a model at the level of the foraging trip for the effects of a fishery on krill predators, using the Adelie penguin (Pygoscelis adeliae) as a model organism.
2. The model has four main components: the description of the spatial and•temporal pattern of krill, ii) the effects of the fishery on the krill, iii) the description of penguin breeding and iv) the indirect effects of the fishery on penguin reproduction and survival. As with all models, there is compromise between the level of tractability and the level of biological detail.The objective is to make relative comparisons of penguin reproductive success and adult survival in the absence or presence of a fishery.
3. The biomass of krill appropriate for the predators (and the fishery) fluctuates from one year to the next according to an age-structured, stochastic recruitment model. We use the model to generate the long-term frequency distribution of krill biomass. Furthermore, we assume that there is spatial-temporal structure, determined by diffusion and advection, to krill availability in relation to the location of the penguin breeding colony. Fishing is assumed to change the spatial and temporal distribution of available krill.
4. We assume that after fledging, offspring survival depends in part upon the amount of krill delivered to them during the feeding periods. We use empirical data to estimate parental and offspring needs and a standard life history model to set the upper limits for expected offspring and parental survival. We assume that parental survival subsequent to breeding depends upon the krill deficiency (relative to needs) accumulated while feeding the young.
5. A sensitivity analysis of the breeding model shows that the predictions are robust to parameters about which little is known, to the functional forms relating krill abundance to offspring and parent survival, and to the rules that parents use to allocate krill to their offspring.
6. We evaluate expected reproductive success (offspring survival) and expected parental survival as functions of the amount of krill captured by the fishing fleet. Over the range of catch in our study, the reductions in reproductive success, are essentially linear functions of krill catch, with slope 1.5. Reductions in adult survival are also linear functions of krill catch, but with slopes less than 1; that is, reductions in reproductive success and parental survival are linear functions of krill catch but not 1:1. The reductions in offspring and parent survival are mainly determined by how long the fishing season lasts and the capacity for harvest, rather than when fishing begins.