Small to meso-scale distribution of Baltic cod (Gadus morhua L.) as resolved by hydroacoustics: Habitat preferences, environmental limits, and resulting implications for stock development
The aim of this thesis is to study the small-scale vertical distribution patterns, habitat preferences and limits for Baltic cod in its major spawning area, the Bornholm Basin. The study is based on hydroacoustic single-target measurements and fish tracking algorithms, which allow to identify individual cod in their environment and to allocate hydrographic parameters to each individual. Additionally, distribution patterns were evaluated to identify the possible influence of climate driven long term trends in abiotic conditions on the Eastern Baltic cod stock. In Chapter I, the applicability of the hydroacoustic single-fish tracking method was confirmed by comparing the results with in-situ distribution data obtained concurrently in other studies from data-storage-tags (DST) attached to cod. It could be shown that hydroacoustics can enhance the datasets provided by DST when it comes to scaling of individual data to population level. Distribution patterns were analyzed for the peak spawning period in two years with contrasting hydrographic conditions (post inflow and stagnation). In both years, vertical distribution of cod was controlled by salinity as upper and oxygen concentration (during stagnation) as lower boundary level, marking suitable and preferred habitat. Temperature had no significant effect on vertical distribution. Spawning cod in both years avoided salinities <11 psu and oxygen depleted layers. A small fraction of cod was distributed in oxygen concentrations of less than 1 ml l-1. It is assumed that these cod undertake short feeding excursions into deeper layers where their prey organisms are distributed. Seasonal changes in vertical cod distribution were analyzed and related to ambient hydrographic conditions in Chapter II to identify the driving mechanisms. In all years observed, cod experienced a loss of suitable habitat with deteriorating oxygen conditions in deep layers during the course of the year. This induced an upward movement of cod. The upper distribution boundary in the upper layers of the halocline remained nearly constant. A GLM model revealed that apart from a significant year-month effect, salinity and oxygen concentrations were the most significant factors affecting cod vertical distribution. This increasingly pelagic distribution is not taken into account in surveys contributing to stock assessment of (Eastern) Baltic cod. Thus, indices derived from bottom trawl surveys in the area might be biased. In Chapter III, climate driven long-term trends in Baltic Sea oxygen concentrations and potential consequences for the Eastern Baltic cod stock were evaluated. Effects of an observed decline in ambient oxygen concentrations in the central Baltic were studied for all life stages of cod from 1951-2007. Based on field distribution data of different life stages and on laboratory experiments it could be shown that negative effects of proceeding oxygen depletion affect all life stages. Cod eggs were and possibly will continue to be frequently exposed to lethal oxygen conditions. Larvae hatching from deeper floating eggs will possibly not be able to undertake vertical migration activities required for initiation of first feeding. Potential settling areas for juveniles deteriorated over 25 % in size during the time the series is representative for. Based on distribution patterns of adult cod and their corresponding abiotic environment, an oxygen induced reduction in gastric evacuation rates was evident leading to reduced overall conditions. Generally, the expected trend in long-term oxygen development will negatively affect all life stages of cod.
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