MEPS 572:209-221 (2017)  -  DOI: https://doi.org/10.3354/meps12128

ABSTRACT: Changes in stock size of a top predator have been found to cascade through the trophic levels, but the mechanism has not been fully clarified. Using data from annual trawl surveys within a fjord during 1988-2015, we present evidence that changes in abundance of cod Gadus morhua and haddock Melanogrammus aeglefinus have an immediate effect on the distribution and a more gradual effect on the abundance of northern shrimp Pandalus borealis. During the first years, the abundance of the gadoids was low and the shrimp was widely spread in the fjord. Following a major increase in gadoid abundance in 2004, the shrimp stock appeared to retreat into the innermost part of the fjord and in the year after the shrimp abundance plummeted. Since that time, the shrimp stock has remained within the same restricted inner area of the fjord while the gadoids have occupied the outer area. A generalized additive model (GAM) indicated that cod had a stronger negative effect than haddock on shrimp abundance in the following year. Stomach content analysis confirmed a greater shrimp consumption by cod than by haddock. Increased bottom temperature may have had an indirect effect on shrimp by attracting more gadoids to the fjord. In tows with large catches of shrimp there were usually few cod and haddock, and vice versa. However, cod juveniles occupied the area in which large shrimp catches were obtained.

KEY WORDS: Predator-prey interaction · Distribution · Abundance · Predation · Aggregation · Anti-predator behaviour