Advances in Fish Tagging and Marking Technology

<i>Abstract</i>.—Environmental variables and interspecific interactions influence fishes’ habitat use. The eurythermic and euryhaline European sea bass <i>Dicentrarchus labrax </i>is increasing in Norwegian waters. We compare diel- and seasonal habitat use and home ranges of this species with Atlantic cod <i>Gadus morhua </i>in the Oslo-fjord, Norway. Individual depth and horizontal positions were obtained using acoustic telemetry. Fish were tracked using both portable and stationary receivers. Both species displayed high site fidelity from June to October. In late October, the sea bass migrated out of the core area whereas the cod remained. Cod home ranges (11.4 ± 5.5 ha) were smaller than sea bass home ranges (42.6 ± 26.1 ha) throughout the study. Home range increased with individual body size in sea bass, but not cod. The two species had differential diel- and seasonal depth use. Sea bass used depths shallower than 4 m during June through September, with diel vertical migrations rarely exceeding 2 m. After September, sea bass sought deeper waters; stabilizing at 11 m in November with a diel migration amplitude of 7 m. Cod stayed deeper than the 9–10 m thermocline in August with vertical migration amplitude of 7 m and moved into daytime depths of 6.5 m with diel amplitude of 4.5 m in December. Sea bass actively selected warmer temperatures and the lowest salinities available, whereas cod avoided temperatures higher than 13°C and salinities below 24 ppt. The differential temperature preference for these species is likely the main driver for the differential depth utilization. A model using air temperatures as a predictor was constructed predicting daytime and night-time depth use. This study provides novel insights to the habitat use of the two species under Nordic fjord conditions—information to be included in upcoming management programs emerging due to the continuing northward colonization of sea bass along the Norwegian coast.