Juvenile gadoid habitat and ontogenetic shift observations using stereo-video baited cameras

The protection of species requires an understanding of their habitat requirements and how habitat characteristics affect their distribution, survival and growth. This need is especially important in areas where anthropogenic pressures can not only have a significant direct impact on the survival of the species but also damage their habitat. The Firth of Clyde in southwestern Scotland was an important commercial fishing area for a variety of demersal fish species up until 1973. However, stocks rapidly declined thereafter and the catch of targeted species ceased in 2005, despite fisheries measures put in place to aid recovery. Changes in the availability and quality of fish habitat are possible explanations for this lack of recovery. Here, we report on stereo baited remote underwater video surveys in the Firth of Clyde between June and September in 2013 and 2014 to determine the habitat of juvenile Atlantic cod Gadus morhua, haddock Melanogrammus aeglefinus and whiting Merlangius merlangus. Habitat predictor variables explored included substratum type, depth, wave fetch, and epibenthic and demersal fauna diversity. G. morhua were most abundant in shallow, sheltered areas composed of gravel-pebble containing maerl. M. aeglefinus and M. merlangus predominated over deeper sand and mud. Ontogenetic shifts in all 3 species were also observed. Relative abundances of G. morhua and M. merlangus were positively related to the diversity of epibenthic and demersal fauna. Our results indicate that spatial conservation measures to benefit demersal fish should be advised by patterns of epibenthic and demersal fauna diversity as well as physical substratum types.