Seasonal windows of opportunity: exploratory trawling for mesopelagic fishes off South Africa’s west coast

Shortages of fishmeal and fish oil and the resultant high prices for these products have intensified commercial interest for potentially harvesting mesopelagic fishes. Data from an exploratory pelagic trawl programme off South Africa’s west coast in 2010/2011 and 2017–2019, largely targeting Hector’s lanternfish Lampanyctodes hectoris, were analysed to investigate drivers of catch per unit effort (CPUE) and to explore operational constraints of a dedicated mesopelagic trawl fishery. Across trips, the mean catch was ∼64 (SE = 5.3) tonnes (t) d-1 and CPUE was 2.7 (SE = 0.2) t h-1, but with high variability. Month was the dominant driver of CPUE, with an elevated CPUE in only 3–4 months of the year. Diel differences (day, night, dawn or dusk), towing speed, fishing depth and bottom depth showed no consistent effects. Catches of mesopelagic fishes were concentrated near the 200-m isobath between Lambert’s Bay and Cape Town, close to the existing fishmeal factories at St Helena Bay. A simple benchmark against local trips made by anchovy purse-seiners indicated comparatively higher trip-level catch rates for anchovy Engraulis encrasicolus (mean ∼157 t d-1) and highlighted throughput and cost disadvantages for a dedicated mesopelagic trawl fishery. The fishmeal and fish oil yield was seasonal, peaking in late summer or autumn and not aligned with the peak in CPUE, negatively impacting the economic viability of this prospective fishery.