Individual behaviour of Baltic cod Gadus morhua in relation to sex and reproductive state

Abstract

Information from data storage tags (DSTs) is conventionally used to infer movement patterns or reveal characteristics (e.g. temperature or salinity) of the environment surrounding tagged fish. Here we link data derived from DSTs with the reproductive state of tagged fish. Individual vertical activity of adult male and female Atlantic cod Gadus morhua L. in the Bornholm Basin was derived from DST measures and related to the individual histologically determined reproductive phase. Spawning migrations were identified by movements towards deeper and more saline waters. No difference was observed between sexes in the timing of the onset of migration and the duration of migration from feeding grounds to the spawning area. While there was no significant difference in duration of the spawning period between females and males, the histological characteristics suggest that females finish spawning before males. Irrespective of gender, vertical swimming activity was most pronounced during spawning, with descents towards the bottom dominating the movements. During spawning, males stayed significantly deeper than females. In conclusion, the present results suggest that initiation of spawning migration and duration of the spawning period differ between sexes, as does the level of activity during spawning events. Not all individuals followed the general pattern; a considerable number of individuals were found to spawn in shallow water in the Arkona Basin, and juvenile fish undertook the migration without spawning.

Evaluation of otolith shape as a tool for stock discrimination in marine fishes using Baltic Sea cod as a case study

“…into SD 25) (Aro, 1989;Borrmann (1985) Berner, 1967;Otterlind, 1985 ). Spawning migrations of cod tagged east of Bornholm (SD 25) to the Arkona Basin (SD 24) also occur (Aro, 1989;Nielsen et al, 2013). These studies suggest that eastern and western 5 Baltic cod stocks co-occur in the Arkona Basin (SD 24).…”

Trophic position derived from amino-acid nitrogen isotopes reflect physiological status of both predator and prey over four decades

“…It is hypothesized that (small) cod are exposed to hypoxic conditions whilst foraging after benthos (Limburg & Casini 2019, Brander 2020). We note that Baltic cod spawning size has decreased over time (Köster et al 2017) and exposure to hypoxia in the fish in this study (~35 cm) could be a result of summer spawning migrations (up to several months in deep, saline but oxygen poor water; Nielsen et al 2013). The stronger correlation between TP-estimates and hypoxia after fat-adjustment of TP (Fig S2) strongly suggests that both growth status and environmental stress influence TEF and TP estimates in this species (Table 1).…”

Use of otolith quality flags to assess distributional dynamics in Baltic cod stocks

“…Sex and length class did not affect differences in the proportion of quality-flagged otoliths. In cod, behavioural differences between sexes are minor (Neuenfeldt and Beyer 2003;Neuenfeldt et al 2007), except for fine-scale patterns during the spawning period (Nielsen et al 2013;Dean et al 2014). Therefore, in any given area, female and male cod most likely experience largely the same hydrographic conditions, which results in similar readability patterns.…”

“…Knowledge of these mixing dynamics and assessing the magnitude of exchange will reduce uncertainty in stock assessment (Paul et al 2013). Several studies in the Baltic Sea have attempted to assign individual cod to a certain stock, for example via tagging experiments (Neuenfeldt et al 2007;Nielsen et al 2013), otolith microchemistry (Heidemann et al 2012), genetics (Eero et al 2012), models (e.g. egg dispersal; Petereit et al 2014) or analysis of otolith shape (Paul et al 2013;Hüssy et al 2014).…”