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

• Main Author: Schaber, Matthias
• Other Authors: Schnack, Dietrich, Gröger, Joachim
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: 2011
• Subjects: thesis; ddc:570; distribution patterns; cod; habitat preferences; habitat selection; hydroacoustic single fish tracking; environmental thresholds; stock development; Gadus morhua
• Online Access: https://nbn-resolving.org/urn:nbn:de:gbv:8-diss-73605; https://macau.uni-kiel.de/receive/diss_mods_00007360; https://macau.uni-kiel.de/servlets/MCRFileNodeServlet/dissertation_derivate_00004058/Matthias_Schaber_DISS_2011.pdf

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 ...