Size selectivity of a novel non-mesh codend (the Modular Harvesting System) in a New Zealand deepwater fishery

Abstract

The Modular Harvesting System (MHS) is an inflatable membrane-like fabric tube with escapement holes that replaces the mesh codend of a trawl, and is designed to reduce damage to catch by providing fish a low-flow, low-turbulence environment that allows them to maintain swimming control and avoid compaction during trawling and haulback. The MHS gear is now used within New Zealand’s most valuable finfish fishery, deepwater hoki/blue grenadier (Merlucciidae, Macruronus novaezelandiae). This work presents the novel research used to estimate the size selectivity of the MHS gear, and to compare this to the selectivity of the conventional mesh trawl. Management objectives required estimates of the absolute size-selectivity of both the MHS and conventional gears, and also an estimate of their relative selectivity. An innovative statistical power analysis was developed to specify the required number of twin-trawl tows and number of fish to measure to ensure the desired statistical power to detect a meaningful difference between conventional and MHS gears. A novel twin-trawl experiment was employed whereby the conventional and MHS gears were each paired with a nonselective control, and then the conventional and MHS trawls were paired. All catch data were simultaneously analyzed in a hybrid selectivity model. The MHS and conventional gears had similar estimated size-selectivity curves, and they were not significantly different.