Individual based model simulations indicate a non-linear catch equation of drifting Fish Aggregating Device-associated tuna

Abstract

Catch per unit of fishing effort (CPUE) is often used as an indicator of tuna abundance, where it is assumed that the two are proportional to each other. Tuna catch is therefore typically simplified in tuna population dynamics models and depends linearly on their abundance. In this paper, we use an individual-based model of tuna and their interactions with drifting Fish Aggregating Devices (dFADs) to identify which behavioural, ocean flow, and fishing strategy scenarios lead to an emergent, non-linear dependency between catch, and both tuna and dFAD density at the ∼1○ grid scale. We apply a series of catch response equations to evaluate their ability to model associated catch rate, using tuna and dFAD density as terms. Our results indicate that, regardless of ocean flow, behavioural, or fisher strategy scenario, simulated catch is best modelled with a non-linear dependence on both tuna and dFAD abundance. We discuss how estimators of CPUE at the population scale are potentially biased when assuming a linear catch response.