Research Document - 2016/106

River Herring Assessment for the Tusket River, Nova Scotia

By Bowlby, H.D., and Gibson, A.J.F

Abstract

This document provides information related to escapement, biological characteristics, commercial fisheries landings, estimated exploitation rates and total mortality of Alewife (Alosa pseudoharengus) and Blueback Herring (Alosa aestivalis), as well as status of Alewife relative to reference points for the Tusket River, Yarmouth County, Nova Scotia. It is the first comprehensive assessment completed by Fisheries and Oceans Canada for river herring in any river in Southwest Nova Scotia.

Similar to sympatric populations in other rivers, Alewife in the Tusket River are larger for a given sex, length or age than Blueback Herring.  Both populations predominantly matured at age 4 and contained a relatively large proportion of repeat spawners.  Alewife made up the majority of the returns, with escapement estimates in the vicinity of 1.7 million fish in 2014 and 2.2 million fish in 2015, while Blueback Herring escapement estimates were in the vicinity of 0.5 million in 2014 and 0.2 million in 2015.  In terms of commercial landings, approximately equal proportions were taken by the dip net and set gill net components of the fishery in 2014, but not in 2015.  After accounting for reporting rates, total landings of Alewife were estimated to range from 2.7 to 2.9 million fish in 2014 and 2.5 to 2.6 million in 2015.  Blueback landing estimates were sensitive to assumed transit times from the estuary to the fishway at Lake Vaughan.  Catch curve analyses in both years indicated total instantaneous mortality rates of > 1.1 for Alewife and < 1 for Blueback Herring.  This is consistent with reduced exploitation on blueback due to run timing relative to the commercial fishing season.  Status of Alewife relative to the reference points derived in the assessment framework indicated that spawning escapement was low (critical or cautious zones) and fishing exploitation rates were either in the fully exploited category or above the removal reference level (overexploited).  The escapement reference points were sensitive to the amount of habitat assumed to be accessible above the Vaughan Dam.  The removal reference points remain unchanged regardless of the river being assessed.  Several sources of uncertainty and their effect on conclusions were evaluated, and short-term as well as long-term research and monitoring goals were identified that would improve the assessment if it continued in future years.

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