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

Abstract

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.

Survival of the fittest: Explanations for gadoid imbalance in heavily fished seas

“…Cod relative abundance was observed to be on average higher than that of haddock and whiting; however, this may have been linked to the shallow depths at which the deployments took place (<48 m). Cod have been observed to occupy shallower waters than haddock and whiting (Elliott, Turrell, et al, ). During both years, a decline in the relative abundance of juvenile cod was observed at the end of data collection, whereas the relative abundances of haddock and whiting were variable, demonstrating either stronger pulse recruitment than cod, or better survival.…”

“…Another factor to take into consideration when considering the gadoid imbalance in the Firth of Clyde, is that the habitats occupied by juvenile haddock and whiting (deeper sand and mud seabed types) (Elliott, Turrell, et al, ), are less vulnerable to damage by mobile demersal gear than that of cod (Collie, Hall, Kaiser, & Poiner, ; Kaiser et al, ). Juvenile haddock and whiting are found over sand and mud substrata, which are also found in large quantities throughout the Firth of Clyde (Elliott, Sabatino, Heath, Turrell, & Bailey, ; McIntyre, Fernandes, & Turrell, ).…”

“…Juvenile haddock and whiting are found over sand and mud substrata, which are also found in large quantities throughout the Firth of Clyde (Elliott, Sabatino, Heath, Turrell, & Bailey, ; McIntyre, Fernandes, & Turrell, ). Cod have been observed in higher relative abundance over fragile maerl and seagrass seabed types, and over more heterogeneous landscapes, within the Firth of Clyde, with ontogenetic shifts in seabed type with increasing size (Elliott, Ahti, Heath, Turrell, & Bailey, ; Elliott, Sabatino, et al, ; Elliott, Turrell, et al, ). The secondary impacts of seabed homogenization from fishing activities (Jennings & Kaiser, ), with cod having more specific habitat requirements, put cod at a survival disadvantage to haddock and whiting.…”

“…Today, Nephrops trawling and scallop dredging still continues, and results in fish mortality through by‐catch (Bergmann, Wieczorek, Moore, & Atkinson, ). The continued mobile demersal activity is of particular importance given that much of the maerl found to occur around the south of Arran has been observed to be in a degraded state (Elliott, Turrell, et al, ). Reduced substratum diversity and quality from decades of demersal mobile gears may therefore be having a negative impact on cod populations (Elliott, Sabatino, et al, ; Elliott, Turrell, et al, ).…”

“…Data were collected within South Arran nature conservation MPA, the Firth of Clyde, south‐west Scotland, at depths of 4.0–47.2 m, between June and September 2013 and 2014 (Figure ). Gadoid sampling took place using three SBRUV frames, as described in Elliott, Turrell, Heath, and Bailey (). From 5 June to 29 September 2013, 80 camera deployments were carried out from a 6.5‐m Rigid Inflatable Boat; however, data were used from 15 July (74 deployments), as before this date no gadoids were observed.…”