Science Advisory Report 2024/047

Assessment of the Status of the Spawning Population of Saint John River Atlantic Sturgeon (Acipenser oxyrinchus)

Summary

• Previous assessments of the Saint John population of Atlantic Sturgeon to support a Non-Detriment Finding under CITES and a Recovery Potential Assessment were based primarily on an evaluation of the size/age structure of the population. These concluded that the population had a healthy age structure with at least 20 age classes, and that the current harvest was sustainable over the short-term.
• Understanding of stock structure (e.g., broad size structure) has not changed from the last assessment. However, in order to answer current and future fisheries management questions, efforts were made to develop and apply a population model.
• Two measures used to manage the modern commercial sturgeon fishery, which are assessed here, are a total allowable annual catch (TAC) of 350 fish (175 males and 175 females) and a closed season in June.
• A two-sex age-structured model was developed using Stock Synthesis 3 (SS3) software and commercial landings, catch data, and biological sampling from the Saint John River (SJR) and Bay of Fundy (BOF). Model outputs were compared to those from a Brownie tag-recovery model.
• In the reference SS3 model, growth, natural mortality (M), and maturity parameters were fixed. Steepness was set at 0.6 because of the late maturity and spawning periodicity of sturgeon. The sensitivity of the model was tested using alternative fishing mortalities (F), percent SJR fish in BOF catch, sex ratio of catch, fishery selectivity, and steepness profiles.
• While this modelling approach allows for stock status to be evaluated by estimating F and Spawning Stock Biomass (SSB) relative to reference points, lack of information on some of the model parameters and lack of confidence in the scaling of the model led to the conclusion that it may be premature to set and evaluate biological reference points from this model for management purposes at this time. However, it was agreed that assessment of fishing mortality relative to fishing mortality reference points (F_ref) would be appropriate.
• Potential fishing mortality and biomass reference points were discussed. A range of spawner per recruit (SPR) values from X = 20, 30, 40, 50, and 60% was calculated, where a higher SPR threshold reflects higher precaution in terms of conservation. SSB at MSY (SSB_MSY) was used as a biomass reference point.
• The mean fishing mortality rate during 2018–2020 was used as the F_benchmark for stock status (relative to F reference points), while the SSB in 2020 was used as the biomass benchmark. All scenarios showed that both F_benchmark/F50% and F_benchmark/F0.1 were less than one. SSB/SSB_MSY was greater than one in 2020 in all scenarios except when steepness equals 0.45. F_benchmark is lower than M estimated from growth parameters.
• Almost all models showed that the stock is in the healthy zone (above SSB_MSY in 2020). Projecting the model until 2030 indicated that the population is expected to remain in the healthy zone with the current TAC of 175 males and 175 females. Only in low steepness scenarios (0.45 and lower values) would the stock size fall below SSB_MSY.
• The June 1-June 30 closure provides sturgeon the opportunity to travel through Long Reach to their spawning location unimpeded and is effective in protecting some females while they enter the river and ripen prior to spawning.
• Various sources of uncertainty were identified that influenced how far this stock assessment model could be developed including catch composition and selectivity of the early historical fishery. Limited information is available to inform population resilience (i.e., steepness), which is needed to inform the development of reference points.
• Impacts of climate change have been observed, for example, two flood years were followed by low river flow in 2018-2020s. Sturgeon has been identified as highly vulnerable to warming water temperatures associated with climate change in the US, and it is recommended that environmental data be included in future assessment development.

This Science Advisory Report is from the March 23-24, 2021, regional advisory meeting on the Assessment of the Status of the Spawning Population of Saint John River Atlantic Sturgeon (Acipenser oxyrinchus). Additional publications from this meeting will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.

Science Advisory Report 2024/047 (PDF, 1,201 KB)

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