Catch rates or catch per unit of effort (CPUE) are used for data-poor exploratory fisheries without integrated assessments in the CPUE by seabed area method to estimate stock biomass in the interim of collecting sufficient tag recaptures. Here, we address the two questions: (1) which unit of effort should be used for catch rates in mixed longline fisheries, and (2) how do different parameters such as gear type, vessel, fishing season, month, bait, fishing depth and area affect estimates of trend and magnitude of catch rates.
Using data from the Ross Sea Antarctic toothfish fishery, we compared effort units including length of line (km), total number of hooks per line, and a combination of total number of hooks per line for autoline and Spanish line and total number of clusters for trotline (hooks/cluster) with Generalised Linear Models (GLMs). The model with total hook numbers was preferred with the lowest Akaike’s Information Criterion (AIC), however standardised catch rates over the fishing season differed little between the models with the three effort units, so the effect of the choice of effort unit on the estimated standardised catch rates is small.
The parameters with the largest effects in the catch rate models were vessel, gear and bait, with vessels showing by far the largest effect size. This confirms previous advice that research fishing is conducted with a high level of spatial and temporal overlap between vessels and gear types to allow for a meaningful standardisation of variables such as catch rates.
To assist in future quality checking of data, we also recommend a new reporting field in the C2 form for the number of droplines per line deployed.