Recruitment variability in vendace, Coregonus albula (L.), and its consequences for vendace harvesting

Abstract

Timo Marjomäki tutkii väitöskirjassaan muikun rekryyttimäärän (=vuosiluokan runsaus ensimmäisen elinvuoden syksyllä tai talvella) vuosien välisen runsaudenvaihtelun ominaispiirteitä ja syitä sekä tarkasteli vaihtelun vaikutuksia kalastukseen ja sen säätelyyn ja toisaalta kalastuksen heijastumista vuosiluokkavaihteluun.

This thesis analyses the characteristics and causes of interannual variability in vendace recruitment and its consequences for, and interactions with, vendace harvesting. The interannual variability of prerecruit mortality was high. The daily mortality during the stage 1-3 weeks after hatching was about 10% and in stage 3 weeks - recruitment about an order of magnitude lower. Yet, the stagespecific total mortalities and their variability were of the same order of magnitude. Thus, both stages had potential for determination of the year-class strength. The variability was largely caused by factors external to the population. The synchrony found in the population dynamics of proximal lakes supported this conclusion. Sparse evidence of compensatory density dependence in mortality was found between hatching and recruitment, but good evidence for compensation was found when the whole period from spawning to recruitment was studied.Considerable model and parameter uncertainty concerning the spawning stock-recruitment relationship existed and methodological biases handicapped the analysis. High larval abundance was required to produce an abundant year-class indicating a positive association between the spawning stock and recruitment in low stock level. Symptoms were found of delayed density dependent mortality induced by a previous year-class causing two year cyclicity. However, tendency for two year cyclicity was also detected for a simulated vendace population under high mortality without delayed density dependence. Observations and conceptual modelling revealed that fulltime commercial fishers diminish interannual yield and income variability by increasing fishing effort during stock decline. Fishing was stopped when the revenue per unit effort went below a certain minimum level. Simulation showed that the strategy of moderately adjusting fishing effort gained compromises of almost as high annual revenue as a constant effort strategy but with lower annual variation and lower risk to spawning stock than the strategy aiming at constant revenue. Successful reduction of variability requires lower mean fishing effort than that producing maximal sustainable revenue. No detrimental effects on pelagic fish stocks associable with fishing were found in a case study comparing zones of a lake with different intensities of trawl fishing. The results thus support high compensation capacity in vendace population dynamics. Means for regulating the number of fishing enterprises based on stock productivity was considered to be a precondition for successful attenuation of yield and revenue variability. Threshold control to secure a spawning stock sufficient to ensure sustainable fisheries was also considered necessary.