Review and evaluation of potential limit reference points for 2J3KL Capelin (Mallotus villosus)

Abstract

Limit reference points (LRP) are an important tool for managing fisheries. In Canada, LRPs are being determined for commercial fish stocks to align with the Fish Stocks Provisions (FSP) of the revised Fisheries Act (R.S.C., 1985, c. F-14). Determining a robust LRP is especially critical for forage fish species due to their commercial value, the critical position they occupy in marine food webs linking the energy produced by plankton to higher tropic levels, and their vulnerability to overfishing. The main purpose of this Research Document is to detail the process for evaluating and determining an LRP for Northwest Atlantic Fisheries Organization (NAFO) Divisions 2J3KL Capelin (Mallotus villosus) (hereafter 2J3KL Capelin), a small, short-lived forage fish in the Northwest Atlantic. Specifically, we overview the relevant legislation and policies surrounding LRPs in Canada, best practices for establishing and evaluating LRPs, the considerations and challenges unique to 2J3KL Capelin and the Northwest Atlantic ecosystem, and the data sets available for this stock. We considered a suite of potential LRPs for 2J3KL Capelin including more conventional approaches based on theoretical or historical proxies for biomass at maximum sustainable yield (BMSY) and unfished biomass (B0), recruitment, and historical trends, as well as newer methods such as multi-indicator, length-based, and ecosystem approaches. Three approaches met the criteria of being feasible, reliable and plausible, and were therefore, considered valid: Brecover based on the 1982 survey, a historical proxy for B0 based on the median value of the highest Capelin biomass in the survey time series (1985–90), and an ecosystem approach based on the capcod model (capcod hereafter). Briefly, capcod predicts the biomass of Northern cod (Gadus morhua; cod hereafter) based on the biomass index of 2J3KL Capelin in the previous year using a bioenergetic-allometric approach. Capcod effectively explained these dynamics (r2 = 0.92) and diagnostics suggest the assumptions of the model were valid. The capcod model was used to determine an LRP for 2J3KL Capelin based on the biomass of 2J3KL Capelin required for cod to reach its own LRP. Of the three valid approaches, the LRP based on the capcod model was clearly the most robust due to the mechanistic approach, the strength of the relationship and diagnostics, and that the resulting LRP is based on an ecosystem approach tying cod to their most important prey, Capelin. The LRP for 2J3KL Capelin was determined to be 640 kilotonnes (kt) and is conceptually consistent with the other major Capelin stocks in the Barents Sea and in waters around Iceland.