The effects of altered freshwater inflows on catch rates of non-estuarine-dependent fish in a multispecies nearshore linefishery

In KwaZulu-Natal on the east coast of South Africa, the commercial and recreational boat-based linefishery is the largest fishery accounting for 1235 t (40%) of the total landed mass there. In particular, the Thukela Banks are where most fishing takes place. Numerous developments ranging from dams to inter-basin transfer schemes either exist or have been proposed for the Thukela River, which is the largest of 17 catchments entering the sea in the region and provides over 40% of the mean annual runoff. Exploratory analyses of the relationships between monthly flows and catch-per-unit-effort were performed using spectral analysis and general linear models on a data set comprising 17 years of monthly commercial catch-and-effort data for 140 species and monthly flow data from 17 catchments. Catchments having a significant influence on catches were those providing the bulk of the runoff volume (75%) reaching the sea. Significant relationships existed between flow and the catches of 14 species, which provided over 90% of the total linefish catch on the Thukela Banks. Time lags between flow events (wet and drought periods) and changes in catch-per-unit-effort corresponded in many cases to age at 50% maturity and/or age-at-first-capture of the species concerned. Under a future maximum-development scenario, corresponding to a 44% reduction in flow from the Thukela River, catches of slinger Chrysoblephus puniceus and squaretail kob Argyrosomus thorpei are forecast to decline by 36% and 28% respectively. These two species currently provide over 50% of the landed mass on the Thukela Banks. Some species should respond positively to a reduction in flow, but any increases in their catches will be negligible, falling within natural variability, and will be insufficient to offset losses of other species because their abundance is naturally low or has been fished down to low levels. National legislation requires that sufficient freshwater flows be set aside to protect ecosystems, so these findings have important management implications, as marine needs for freshwater flows have not previously been addressed.