The mesh size effect: counting long thin fish in seagrass

Seagrass meadows and the often poorly described ichthyofauna that reside within this habitat are threatened globally. Small-bodied fish in seagrass habitats are often of conservation concern due to their vulnerable life-history characteristics, depauperate distribution and abundance data and importance to artisanal fisheries. We employ a novel approach simultaneously deploying concentric trawl nets comparing mesh sizes that capture cryptic within-canopy species from Posidonia spp. meadows in Shark Bay and Ngari Capes Marine Parks, Western Australia. A discrete species assemblage passed through the 10 mm trawl net and were retained on the 2 mm net at both locations without effecting species richness estimates. Fish size distributions within net fractions differed between locations; fish retained by the 2 mm net at NCMP were much longer than those in the >10 mm net-fraction at that location, and the 2−10 mm portion of the SBMP catch. Of pipefish species globally, 30 % are identified as data-deficient by the IUCN. In NCMP 46 % of fish in the 2−10 mm fraction were pipefish and only 4 % of the >10 mm fraction, whereas pipefish were just 0.4% of individuals recorded in the smaller mesh size in SBMP. In both locations, seahorse species were only captured in the larger net size. These results demonstrate that small differences in net mesh-size can drastically alter catch compositions in seagrass habitat. Accurately surveying syngnathids of conservation interest will require the use of a net mesh that selects fish based on body shape rather than length, an important consideration regarding data-deficient pipefish species. Furthermore, fisheries and research by-catch may be minimised by using mesh sizes that avoid prominent syngnathids when net-fishing in seagrass.