Research Document 2017/065
Framework Assessment of the Offshore American Lobster (Homarus americanus) in Lobster Fishing Area (LFA) 41
By Cook, A.M., Cassista Da-Ros, M., and Denton, C.
Abstract
The Lobster Fishing Area (LFA) 41 offshore Lobster fishery has been active since the early 1970s and is currently the only Total Allowable Catch (TAC) based Lobster fishery in Canada. The TAC has been set to 720 t since the mid-1980s without change, despite increases in survey biomasses. The fishery currently has 8 licenses, which are owned by a single corporation and are fished from a single vessel.
This stock assessment framework updates that of Pezzack et al. 2015 by exploring the impact of stock boundaries on indicator trends as well as expanding the suite of indicators used to describe the ecosystem and Lobster stock and exploring options for defining stock status and reference points.
Four multispecies trawl surveys conducted by two agencies, Fisheries and Oceans Canada (DFO) and the National Marine Fisheries Service (NMFS), occur within LFA 41 and adjacent areas. Each of these surveys provides indices of biomass and abundance, size frequency, sex ratio, distribution and environmental variables. Six at-sea observed trips are conducted each year, which provides further information on by-catch profiles and Lobster size and sex information.
Time series’ of a suite of standard indicators including total abundance, median and maximum size, mature and immature sex ratio, patchiness of distribution, area occupied, abundance of large females and recruit abundance were used to describe the changes in the LFA 41 Lobster stock over time. Additionally, ecosystem indices including predation, bottom water temperature, the Atlantic multidecadal oscillation (AMO) were provided to describe some of the external factors which may impact Lobster productivity. All indicators were combined and ranked through a modified principle components analysis to display the coherence in indicator trends over time. Overall, patterns suggest decreasing median and maximum size of the Lobster stock over time, as well as decreasing predation pressure and increasing abundance, distribution, bottom temperature and AMO.
The LFA 41 stock has never been assessed with a quantitative stock assessment model. Here, a biomass-dynamic model fit through Bayesian state-space methods was attempted for this stock. The carrying capacity parameter was not well defined and was influenced by the median of the prior distribution. The inability to define parameters was partially due to the lack of contrast in the data from the constant TAC despite increased survey biomasses. It was concluded that this quantitative model was currently inappropriate to provide estimates of stock status and reference points, but should be explored in future.
Data driven primary indicators and methods to develop upper stock and limit reference indicators (USI and LRI, respectively) were described. Several options for reference indicators were explored for each of the four surveys covering LFA 41, with the recommendation of a USI being based on survey biomasses from the high productivity period and a LRI defined similar to Brecover. Although each of the surveys had very similar trends in commercial biomass, with recent years being the highest on record, the recommendation was to continue using all four surveys and evaluate the overall stock status based on the status of 3 of the 4 survey trends. Specifically, 3 of 4 survey trends would need to be below their respective USI’s in order for the stock to be considered in the cautious zone. Similarly, 3 of 4 survey trends would need to be below their respective LRI’s in order for the stock to be considered in the critical zone
Methods to describe removal references were described; however, due to the stable TAC and currently increasing biomass, the impact of harvesting on stock status was not readily determined. The recommendation was to not provide a removal reference based on the information currently available.
A reproductive potential primary indicator was developed along with boundaries relating to stock productivity. Reproductive potential has long been considered an important component of Lobster stock productivity that is in need of protection, and although this may be more important for inshore Lobster fisheries that are largely recruitment fisheries, the LFA 41 Lobster stock is predominated by large female Lobsters. Removal references and stock status zones were not defined for the reproductive potential indicator; however, it was recommended to be tracked independent of the other indicators as changes may be indicative of the state of future stock productivity.
Analysis of bycatch data and research recommendations were also included in this framework document.
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