Fishery discards in the irish sea exhibit temporal oscillations and trends reflecting underlying processes at an annual scale

Abstract

Non-inclusion of discard data in stock assessment can lead to underestimation of biomass and fishing mortality; this is of particular concern if there have been changes in discard practices over time. Although variability in space and time is a well-documented feature of discards, the temporal dynamics of the practice has received little detailed attention. The aim here is to characterize the temporal patterns of discarding practices in the Irish Sea (ICES Division VIIa) from 1994 to 2008. Trend and seasonality were explored in discards per unit effort (dpue) of haddock (Melanogrammus aeglefinus), whiting (Merlangius merlangus), and cod (Gadus morhua) through Bayesian harmonic regression (HREG) models. The HREG models reveal discarding of all three species in annual cycles, with a peak in the second quarter, perhaps reflecting species biology or fisher behaviour, or both. The dpue of cod cycled around a constant level throughout the observation period, but whiting and haddock dpue increased.