School characteristics of mesopelagic fish at South Georgia

• Myctophid fish formed regular schools at South Georgia and their schooling characteristics varied inter-annually.
• Schools were distributed on-shelf in some years, but off-shelf in others. They also appeared to be separated by bathymetry.
• Offshore schools occurred higher in the water column than onshore schools, and they were longer, thinner, and acoustically weaker.
• This might be related to (a) predatory pressure (b) oceanographic variation, or (c) habitat partitioning of different species/life stages.
• Our study provides new information on the ecology of myctophids of use to predator foraging models and ecosystems studies.

In this paper, we describe the variation in myctophid schools characteristics at South Georgia using multi-frequency acoustic data collected annually between November 2007 and January 2012. We studied the relationship between the proximity of land and the distribution and schooling characteristics of myctophid fish. We also examined patterns in schooling behaviour in relation to oceanographic data. Fish schools were identified using a dual-frequency dB identification method (Sv120−38 kHz), where negative Sv120−38 kHz is indicative of gas-bearing organisms, such as swimbladdered fish. Available net data were used to provide information on the meosopelagic fish community in the region. School morphometrics (e.g. length, thickness, area) data were extracted and pooled according to their distance from the shore (0–30 km, 30–60 km, 60–90 km, 90–120 km). A total of 578 schools were detected in the survey region and there was high variation in school backscattering strength (NASC), length, height, perimeter, depth and horizontal distribution between surveys, including distinct inter-annual variation in these parameters when surveys were conducted in the same season (2009–2012). Schools were distributed predominantly on-shelf (0–500 m isobaths) during some summer surveys (2007 and 2012) and predominantly off-shelf on others (>500 m isobaths; 2009, 2010 and 2011). There was also evidence of bathymetric separation in the horizontal distribution of schools. Schools observed in the late-season in 2008 had the greatest NASC, largest height and the deepest depth distribution. Most schools occurred at depths between ∼60 and 300 m, but they did not occupy the same water masses during each survey. Schools became progressively thinner, shallower and acoustically weaker with increasing distance to land, whilst school length and perimeter increased correspondingly. The change in fish schooling behaviour between environments could be a response to a combination of local predatory threats over short spatial/temporal scales and differences in oceanographic conditions, such as current velocity. The trend could also be evidence of spatial habitat partitioning of myctophids, with different schooling species, or different life-stages, orientating along a bathymetric gradient at South Georgia.