Examining cannibalism in relation to recruitment of silver hake Merluccius bilinearis in the U.S. northwest Atlantic

Abstract

Cannibalism can be an important feature for several fish species. From an ecological perspective, studying cannibalism in the northwest Atlantic fish community usually results in examining silver hake Merluccius bilinearis population dynamics. From a population dynamics (and hence fisheries assessment) perspective, silver hake pose some unique challenges and have defied easy, standardized approaches for evaluation of stock dynamics. Thus, here we propose to examine one facet of silver hake dynamics in light of their known cannibalistic propensity. We show for the two U.S. northwest Atlantic stocks that the percent diet composition of silver hake in silver hake is quite high and has been quite consistent over time. We also explore the size composition of cannibalized individuals as well, noting that the 0–10 cm grouping is largely cannibalized by the 20–40 cm group, which is itself cannibalized by 40–60 cm group. From this, we fit stock-recruitment models (both Ricker and Beverton–Holt) that were typical and excluded cannibalism, that included cannibalism as an additional factor, and that changed the definition of the spawning stock biomass (SSB) from 20+ to those fish that are >40 cm. This latter shift resulted from the observed offset of recruitment production versus copious cannibalism as seen in the middle size group. Our results show that either the inclusion of cannibalism directly, or accounting for cannibalism indirectly by using a larger cutoff for the definition of SSB, are better model fits for both stocks than ignoring cannibalism for this species. We discuss the broader implications of including cannibalism for this species, for other cannibalistic species, for changes in how we consider recruitment—particularly those species managed with recruitment-based reference points— and for how these might alter perceptions of stock dynamics.