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Research Document - 2006/041

Assessment of Status and Recovery Potential for Striped bass (Morone saxatilis) in the Southern Gulf of St. Lawrence

By Douglas, S.G., G. Chaput and D. Caissie


The COSEWIC’s recommended "Threatened" listing for striped bass in the southern Gulf required that DFO’s Section 73 permitting framework be applied to determine if incidental harm would jeopardize survival or recovery of the species. Three primary indicators were explored to determine the status of striped bass in the southern Gulf. First, mark-recapture experiments conducted on striped bass returning to the Northwest Miramichi River to spawn indicated an average of 22,000 mature individuals each year since 2001. This represented a modest increase from a low of approximately 4,000 annually in the 1998-2000 period but not as high as the peak level of 50,000 spawners observed in the mid 90’s. Secondly, analyses of the striped bass bycatch in the gaspereau fishery of the Northwest Miramichi River indicated an average of 84 bass per net per day over the last 5 years and closely correlated with estimates of population size derived from mark-recapture experiments. Lastly, spawning success measured from catches of young-of-the-year in the fall open-water smelt fishery (1991-1998) and beach seine surveys (2001-2005) were weakly correlated with spawner abundance estimates and suggests that year-class success can be determined by environmental factors. Secondary indicators of status such as the truncated and unchanged age and size distributions for spawning striped bass since the early 90’s supported the high natural mortality estimate of 0.54-0.59 derived for this population after the commercial fishery closure in 1996. Tagging studies continue to define the whole of the southern Gulf as the area of occupancy for this population of striped bass.

A discrete life history model was used to propose reference levels for recovery of the southern Gulf population. The recovery objectives parallel the precautionary approach benchmarks of critical, cautious, and healthy zones. We propose an Sopt value of 21,600 spawners as the recovery limit for southern Gulf striped bass and the 50%SPR value of 31,200 spawners as the recovery target, the latter being the value for managing any directed fisheries. The Seq value (spawners at replacement in the absence of fisheries) was estimated at 63,000 fish. We discuss the need to implement compliance rules and suggest that a 6 year sliding window may be appropriate with the objective of exceeding the recovery limit in 5 of 6 years. Under present conditions, including bycatch of YOY and continued illegal removals of adult bass, there is a low probability (18%) that the southern Gulf striped bass will be above the recovery limit by 2015. If the total mortality on adults is reduced to Z = 0.6 from the current condition of Z = 0.8 and YOY bycatch is eliminated, there is a greater than 95% chance that the population will be above the recovery limit by 2015.

The Northwest Miramichi River remains the only confirmed spawning location for striped bass in the southern Gulf. Because striped bass occupy all of southern Gulf but yet continue to show high fidelity to the Northwest Miramichi, the colonization or establishment of new spawning locations may not be a realistic recovery objective.

Quantitative estimates of mortality were not possible for each of the major threats believed to be limiting the rebuilding of this population. Illegal harvests are believed to be the single greatest cause of mortality for the population. Total accumulated mortality does not seem to jeopardize the survival, but under present conditions, recovery above the proposed limit is unlikely.

Mitigation measures are discussed. Recovery efforts for southern Gulf striped bass should focus on reducing adult mortality and YOY bycatch and protecting the striped bass habitat and spawning grounds of the Miramichi system.

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