Science Advisory Report 2021/018

Reference points for Striped Bass (Morone saxatilis) for the southern Gulf of St. Lawrence population

Summary

• Striped Bass (Morone saxatilis) of the southern Gulf of St. Lawrence (from western Cape Breton Island NS to the north shore of the Gaspe Peninsula QC) is considered to be a single biological unit for the purpose of this assessment and the derivation of reference points.
• This Striped Bass population has been annually assessed since 1994 and its life-history characteristics, including size-at-age, length-weight relationship, and sex ratio of spawners are well defined.
• An age structured population model with an underlying Beverton-Holt stock and recruitment relationship (from eggs to age-0) was used to model the population dynamics of Striped Bass. The model uses a combination of estimated and assumed life history parameters, most of which are specific to this population. Seven models were evaluated; the two preferred models are presented in this report.
• Monitoring and stock assessment data of the spawners in the Northwest Miramichi River, the annually predictable and confirmed spawning location that produces recruitment, are considered representative of Striped Bass in the southern Gulf and appropriate for deriving reference points.
• The near monotonic increasing trajectory of the population abundance from its low point in the late 1990s at less than 5,000 spawners to the highest abundance in the late 2010s of over 300 thousand spawners provides limited information to unequivocally define the maximum potential population size. A conclusion from the population modelling with the available data is that the maximum abundance of this population has not yet been realized.
• Reference values are defined in terms of eggs and converted to equivalent numbers of spawners to aid in interpretation.
• The number of eggs that equate to 80% Bmsy (80% of the spawning stock biomass that produces maximum sustainable yield) is proposed as the Upper Stock Reference (USR). There is no model consensus for the USR value. An Upper Stock Reference value of 54.3 billion eggs, equivalent to 720 thousand spawners, is the lowest value of two models retained (versus 1.2 million spawners for the other model).
• The number of eggs that result in half of Beverton-Holt carrying capacity is proposed as the Limit Reference Point (LRP). There is no model consensus for the LRP value; 17.3 billion or 30.0 billion eggs depending on the model, equivalent to 330 to 560 thousand spawners. Based on the trajectory of this population over the relatively short period of assessment, maintaining spawners above 330 thousand fish should be sufficient to avoid serious harm.
• The status is presented in terms of estimated eggs from spawners and perspectives on status are model dependent. The highest estimated spawner abundance of approximately one million fish in 2017 was approximately at the USR or in the cautious zone depending on the model. Otherwise, the status was either below the LRP in all years except 2017, or below the LRP until 2015 and in the cautious zone since 2016.
• Fisheries management actions were responsive to the decline and rebuilding of the Striped Bass population, beginning with the closure of all directed fisheries for Striped Bass in 2000, followed twelve years later with the re-opening of the Indigenous FSC fisheries in 2012, and the retention recreational fisheries in 2013. This increased fisheries access occurred as the stock abundance was on an increasing trajectory of abundance, moving into the cautious zone.
• Prey of Striped Bass in May and June in the Miramichi River include Rainbow Smelt, gaspereau, occasionally Atlantic Salmon smolts, as well as several other fish and invertebrate species. Alternate reference levels to address the multiple species concerns related primarily to predation by Striped Bass on these prey species cannot be determined at this time based on the available information.
• The most important assessment and management uncertainty is the lack of comprehensive recreational fisheries catch and harvest data. In absence of such data, assessments of the dynamics and robustness of this population to fishing and environmental variation cannot be provided.

This Science Advisory Report is from the regional advisory meeting of November 23-25, 2020 on Reference points that conform to the Precautionary Approach for the Striped Bass population of the southern Gulf of St. Lawrence. Additional publications from this process will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.

Science Advisory Report 2021/018 (PDF, 1,352 KB)

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