Science Advisory Report 2022/014

Stock status of Alewife and Blueback Herring (Gaspereau) in the DFO Gulf Region

Summary

• Alewife and Blueback Herring are anadromous species of fish collectively referred to as gaspereau or river herring.
• Alewife and Blueback Herring in individual rivers are considered to be different stocks. The two species have different migration timing and spawning areas and therefore, stock assessments are conducted at river-specific scale and separately for each species.
• In the Gulf Region, reported commercial landings of gaspereau from DFO Statistics Branch have declined over the time series from 1917 to 2019. These are known to be incomplete and may not reflect abundance.
• There are very few region-wide indicators of abundance. At the regional scale, the bottom-trawl surveys show a decline in catch rates of gaspereau over time although it is unclear how well the survey indices reflect abundance.
• Indicators of stock status for Alewife and Blueback Herring are available in the Margaree River. Catch per unit effort in the commercial gaspereau fisheries peaked in the 1980s and has fluctuated around a mean of 61 kg/hour since the last management changes were implemented in 2001.
• Over the sampling period of 1983 to 2019, the average size of Alewife and Blueback Herring in the Margaree River commercial fisheries declined 41% and 53% by length and 10% and 13% by weight, respectively.
• Variations of age-structured stock assessment models were used to evaluate the status of Alewife in the Margaree River. Estimates of spawner biomass varied between 26 metric tonnes (t) to 1,370 t from 1983 to 2019. Exploitation rates varied from 0.29 to 0.89 during the same time period.
• Population-specific reference values are developed for Alewife and Blueback Herring. The limit reference point (LRP) is set equal to the spawner biomass that produces half the maximum recruitment (K). The VPA model estimated LRP to be at 113.9 t (more conservative) whereas the SCA model estimated LRP to be 64.3 t. The upper stock reference (USR), is set equal to the biomass of spawning alewife at maximum sustainable yield (B_msy). The VPA model was more conservative with a USR of 590.8 t whereas the SCA model gave of USR of 276.0 t. Low abundance during the time series causes uncertainty in their estimated values. Both reference points will need to be re-assessed if stock size increases.
• Although the values of abundance and reference points differ among the models, the estimated biomass of Alewife in the Margaree River before the fishery has generally been in the cautious zone since the mid-1990s and after fishing, spawning escapement has been in the critical zone in over half the years.
• The target removal rate is set at U_90%msy (VPA model: 0.33 and SCA 3 model: 0.41). The maximum removal rate is defined as U_msy (VPA model: 0.51 and SCA 3 model: 0.60) and should not be exceeded. Both the target and maximum removal rates only apply in the healthy zone.
• Although the absolute values of the realized exploitation rates and the removal rate reference points differ between the two models, the estimated exploitation rates have generally been above the target removal rate in all years and above the maximum removal rate references more than half the time from 1983 to 2019.
• Indicators of stock status for Alewife and Blueback Herring in the Southwest and Northwest Miramichi River were collected from DFO research trapnets upstream of where fishing occurs. Catches of both species have declined on the Southwest Miramichi River whereas catches on the Northwest Miramichi River have been variable.
• The commercial gaspereau fisheries in the Miramichi River have not been sampled since 2000. Indicators of stock status are inferred from catches at research trapnets. Catches declined on the Southwest branch during 1994 to 2019 and were highly variable on the Northwest over the 1998 to 2019 time series. When last assessed in 2000, the exploitation rate in the Miramichi River gaspereau fisheries were high and above the removal rate references defined for those stocks.
• Catch curve analysis of catches at Miramichi research trapnets indicate that exploitation rates remain high on that river and are most likely above the removal reference rate for other gaspereau stocks.
• Assessment of the status of other Alewife and Blueback Herring stocks in DFO Gulf Region was not possible due to data limitations. Sampling of the fisheries other than the one in the Margaree River has not been conducted since 2000.
• Gaspereau fisheries are conducted on a large number of rivers in the Gulf Region and systematic sampling of the fisheries catches has only been conducted on one river. In order to assess the status of gaspereau stocks in the region, additional effort will be required to sample these fisheries on an intermittent and rotational basis, and the development and validation of other indicators of status such as by examination of age composition and catch curve analyses will be required.

This Science Advisory Report is from the April 20-21, 2021 meeting on the assessment of the status of gaspereau stocks of the southern Gulf of St. Lawrence. 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 2022/014 (PDF, 1523 KB)

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