Abstract
Despite research and public scrutiny over recent decades, discarding continues to be an issue for trawl fisheries. Previous research demonstrates that environmental, biological, operational, legislative and socioeconomic drivers affect a fisher’s decision to discard an organism. Therefore, the reduction of fishery discards requires a better understanding of fishery-specific drivers. Despite considerable research and mitigation, further work is required to reduce discarding to acceptable levels (currently ~ 50% in Australia). To better understand the drivers of discarding, this study used a modelling approach to determine environmental and operational factors that drive discarding in the New South Wales (NSW) ocean prawn trawl fishery (OPT). Further, the study investigated the relationship between the discarded number of individuals from all functional species groups (i.e. elasmobranchs, crustaceans and fish combined) and the retained catch weight. This model was also run on just fish partly due to their disproportionally high contribution to the discard assemblage (e.g. 76% of all species or higher taxon) and importance (e.g. to the ecosystem and fisheries). The results quantified relationships of environmental and operational drivers of discarding and the relationship of fish discarding and retained catch weight was found to be linear. However, the identified relationships appear complicated and, whilst an important first step, more work is required to identify all drivers influencing discarding practices. We, in combination with previous research, suggest implementation of effort quotas may be a suitable management initiative to reduce discarding and its impact; at least whilst more research is conducted to better understand this complex process. Furthering our understanding of discarding is urgent given its global impact and the rate of discarding in the OPT.
Highlights
ObjectivesUnderstanding environmental and operational factors that drive discarding could aid in its reduction via informed management
Observers were present on 422 overnight ocean prawn trawl fishery (OPT) trips for 1 266 hauls (~ 3 hauls per trip, Table 1)
There was a negative north to south gradient in the number of trips observed reflecting the amount of fleetwide OPT effort (Fig 1)
Summary
Understanding environmental and operational factors that drive discarding could aid in its reduction via informed management. We investigated the following hypothesis: 1) Environmental (spatial, temporal and habitat (depth)) and operational (retained catch weight, trawl speed, track complexity and engine capacity) drivers do not affect discard numbers. To explore the relationship between the volume of retained total catch and (1) discarding of fish, and (2) discarding of the study species groups combined (elasmobranchs, crustaceans and fish), we tested the following hypothesis: 2) Retained catch and discard rate of fish, and all species groups combined are not correlated
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