Thermal conditions and age structure determine the spawning regularities and condition of Baltic herring (Clupea harengus membras) in the NE of the Baltic Sea.

Elor Sepp,Heli Shpilev,Tiit Raid,Timo Arula

doi:10.7717/peerj.7345

Abstract

Baltic herring (Clupea harengus membras) is a total spawner with a group-synchronous ovarian organization. Age polymodality in total spawners is considered an important factor in assuring that a strong population is sustainable under an intensive harvesting regime and different climatic conditions. In the present study, we investigated the seasonal and inter-annual variation in spawner age structure and the effect of preceding winter thermal conditions on the start of the herring spawning and larvae retention period. Herring spawning season in the Gulf of Riga starts up to six weeks later after colder winters compared to milder winters. Significantly older individuals dominated at the beginning of the spawning season, and thus herring mean age gradually decreased towards the end of the spawning season from 1999–2015. On an annual scale, this pattern was obvious after cold winters, while after mild winters the pattern did not continue, indicating a more homogenous maturation cycle and spawning period, despite the age and size of the herring population in mild winters. Further, herring condition factor was studied in relation to age and spawning season following different winter thermal conditions. Young, 2- and 3- year old first-spawning herring experienced significantly lower conditions after cold winters compared to older ages, indicating an age-dependent effect of preceding winter on herring maturation cycle, condition and spawning time.

Highlights

Age structure of spawning populations, spawning seasonality, and migration to spawning grounds have been widely studied in relation to fishing regulations for population conservation and climate variability (Lambert, 1987; Sims et al, 2004; Sissener & Bjørndal, 2005; Margonski et al, 2010)
For Baltic spring spawning herring (Clupea harengus membras), there have been no studies focusing on the age structure of spawning population, such material has been collected with high precision for decades for routine stock assessment
Earlier larval first appearance corresponded to an earlier start to spawning season

Summary

Introduction

Age structure of spawning populations, spawning seasonality, and migration to spawning grounds have been widely studied in relation to fishing regulations for population conservation and climate variability (Lambert, 1987; Sims et al, 2004; Sissener & Bjørndal, 2005; Margonski et al, 2010). Within the same spawning season, many different spawning groups of herring exploit same spawning ground, migrating to spawning grounds in ‘‘spawning waves’’. Predictions of such spawning waves provide resource managers grounds to plan commercial fishing to conserve certain age classes (Sissener & Bjørndal, 2005). Fish tend to have certain adaptation plasticity in the timing of life cycle events such as reproduction and in the length of spawning seasons as a response to environmental cues (e.g., photoperiod and temperature, Geffen, 2009)

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