Survey based estimation of consumption: spatial and seasonal aspects of cod predation on shrimp

Several assessment reports on the shrimp stock in the Barents Sea have concluded that any assessment model introduced should include the cod consumption on the shrimp stock. However, the shrimp stock does not show obvious response to the cod consumption. The method for estimating the cod consumption has been evaluated previously. These indicate that the cod consumption is overestimated both as a whole as well as in particular years. Still the shrimp stock does not respond directly on the cod consumption. We here study if the lack of overlap and different season of data collection has influenced the consumption estimate. This is now possible as the winter survey and the ecosystem survey sample cod, cod stomachs and shrimp. This should enable us to get consumption estimates and shrimp index estimates that are directly comparable and applicable in shrimp stock assessment. As the assessment relies on historical data the historical data should be properly described and evaluated. In this document we use and present historical data on shrimp abundance, cod stomach data, and estimated consumption to study spatial and seasonal variation in shrimp consumed by cod. The modified estimate had the same temporal signal as the unmodified estimate, and thus will not provide a better fit in the assessment model. Cod consumption of shrimp might differ with season, but unfortunately, the survey coverage by season has varied from year to year. We tested for seasonal effects on consumption in years with good coverage in summer and winter. We did not find any consistent seasonal effect. Introduction Cod is considered an important predator on shrimp in most areas and imperative to any shrimp assessment model for the Barents Sea stock (Anon 2005). However, whereas there is little doubt that cod can consume considerable amounts of shrimp, the quantitative implementation of this observation in an assessment model might not be so straightforward. Currently, there are no agreed upon quantitative assessment of the Barents Sea shrimp stock. However, in recent years, IMR has performed an assessment based on logistic stock-recruitment function and Bayesian interference (Hvingel 2006). However, the shrimp assessment model performs less well when including the shrimp biomass consumed by cod (Hvingel 2006, Aschan et al. 2006), even though the estimated predation from cod on shrimp is of considerable magnitude and many times higher than the catches (Figure 1). This is also the case when using a modified approach to estimate consumption, based on survey design (Aschan et al. 2006 and below (Figure 1). In brief, cod abundance and cod consumption just includes more “noise” and makes the parameter estimates in the assessment model less precise (Aschan et al. 2006, Hvingel 2006). Hvingel (2006) stated that “an underlying spatial structure of the estimates of shrimp stock size and consumption by cod need to be resolved”. This should be done in order to include cod predation into shrimp assessment. Unfortunately, until recently, monitoring of cod and shrimp in the Barents Sea, have been conducted on separate cruises (the Norwegian shrimp cruise 1981-2004, and the IMR winter survey for demersal fish 1981-), with different area coverage, making it difficult to compare spatial structure and distribution of cod and shrimp directly, and to study overlap (Figure 2). Furthermore, cod stomach data has only (with a few exceptions) been collected on cruises monitoring cod. Fortunately, from 2005 on; data on shrimp and cod abundance and cod diet are sampled on the Joint IMR-PINRO ecosystem survey, covering the whole of the Barents Sea and Spitsbergen area (Figure 2). Also, from 2006 on, the winter survey for demersal fish in the Barents Sea also records shrimp abundance. This should make it easier to get consumption estimates and shrimp index estimates that are directly comparable and applicable in shrimp stock assessment. However, assessment relies on historical data. Therefore,