Research Document 2021/066
Units 1 and 2 Redfish Management Strategy Evaluation
By McAllister, M.K., Duplisea, D.E., Licandeo, R., Marentette, J.R., and Senay, C.
Abstract
A Management Strategy Evaluation for the Atlantic redfish fishery in Fisheries and Oceans Canada management Units 1 and 2 in the Gulf of St. Lawrence and the Laurentian Channel was carried out. Three conservation objectives and four fishery objectives were formulated based on discussions in meetings between managers, scientists, industry members and other stakeholders. 18 performance metrics were defined to quantify how well candidate management procedures met each of the eight management objectives. A set of 18 operating models were formulated to evaluate the robustness of candidate management procedures to key uncertainties affecting the evaluation of management options. The operating models were age structured and fitted separately to data for the two redfish species, i.e., Sebastes mentella and S. fasciatus.A Bayesian approach to model fitting was adopted in which informative priors were applied for the steepness stock-recruit parameter for the Beverton-Holt stock recruit function, and historical deviates for cohorts that early research had concluded were strong ones. The models were fitted to time series of bottom trawl survey indices of abundance and length composition from Units 1 and 2, and length composition records of retained redfish catches in Units 1 and 2. Some of the key axes of uncertainty considered in formulating the operating models included uncertainty over fishery vulnerability-at-age, life history parameters, e.g., the rate of natural mortality and growth, the future time series of cohort strength, the magnitude and species composition of historical catches, and plausible values for the steepness stock-recruit function parameter. A set of six core operating models were judged to have higher credibility than the set of 12 stress test operating models which were still credible but according to technical and other considerations judged to be less so. Candidate management procedures were required to pass threshold values for performance metrics for the conservation objectives and were ranked according to the fishery performance metrics for the six core operating models. The management procedures (MPs) were model-free and included three key components. The primary one was a harvest control rule (HCR) which specified a catch limit based on the three-year trailing average of Unit 1 trawl indices. The second specified the start-year for the HCR which varied between 2018 and 2022. The third specified whether a maximum cap to catch limits was to be applied and what the cap should be in each future year. If the MP was capped, a ramp cap was initially applied from the initial year to some intermediate year in the 40-year horizon. Here, the cap was increased in successive years, with the anticipated (1) recruitment to the fishery of the large 2011-2013 cohorts and (2) increases in industrial capacity. A maximum cap was then applied at the end of the ramp. For the conservation objectives, capped MPs outperformed uncapped MPs under most of the core and stress test operating models. Average catches retained and interannual variability in catches were much higher for uncapped than for the capped MPs. Four of the uncapped MPs failed on one of the stock conservation objectives for one of the core operating models. Under the less optimistic core and stress test operating models, for example, historical discarding is lower than that in the base case, there were a number of MPs that passed all stock conservation objectives and provided average future catches of about 24 kt over the next 10-20 years. Projected median total allowable catches (TACs) over the next five years from most MPs were sufficiently low as to constitute low conservation risk to the stocks. Stock conservation objectives that aim to reach in 10 years and then maintain the SSB of both species in the Healthy Zone were met with high probability in all tested MPs. The abundant small fish (<22cm) associated with the 2011-2013 cohorts results in predicted catches from all candidate MPs failing to meet the requirements of the Small Fish Protocol in 2018 and 2019. Small fish are expected to remain abundant in the catch until 2020. Exploratory analysis indicates that the performance of MPs with respect to three conservation objectives was improved and total catches were increased in simulations that assumed that the two species of redfish were perfectly distinguished in fishery catches, enabling species specific TACs.
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