FIS007: Survival of post-catch Nephrops A
Start Date: Jan 2015
End Date: Sep 2015
Main Contractor(s): University of Stirling, SAMS
Other Sponsor(s): European Fisheries Fund
Extended Title: Post-catch survivability of discarded under-sized Norway lobsters (Nephrops norvegicus): Towards a regional and ecosystems-based approach.
Main Research Category: Discard Reduction - Technical
View Final Report
View supplementary documents one and two
Summary of Project Outcomes
- Discarding of Nephrops by a typical Clyde trawler operating a tubing fishery, and physiological recovery of discarded Nephrops, was studied in three trials in late winter (February), early spring (March) and summer (June) 2015. In addition, the behaviour of discarded Nephrops was studied in July 2015.
- From the three trials conducted (8 tows in total) around 30% of the catch (by numbers) was observed to be discarded because the animals were too small, were damaged, recently moulted or were visually infected with Hematodium spp..
- The ‘discard’ fraction of the catch was mainly composed of prawns which were larger than the minimum permitted landing size (MCS ≤20 mm).
- The composition of the discard fraction also varied seasonally. In winter and early spring (February and March trials) the discarded fraction contained more males and more visually infected animals than in early summer (June). In June, discarded animals were predominantly females of a bigger size than their counterpart discarded males, they were in a soft condition (indicating that are close to or have recently moulted) and contained mature and developed gonads.
- Taken data from all tows, 87% of the discarded Nephrops showed no observable damage after trawling.
- Overall, data from survival experiments indicated that 95% of discarded Nephrops were alive after 48 hour of recovery period in tube-sets.
- The use of tubes for recovery may have led to some additional physical damage of the prawns which might have affected around 5-6% of the test animals (averaged across all 8 tows).
- The physiological recovery potential of discarded Nephrops was high especially considering the biochemical condition of the animals shortly after being taken from the trawl (elevated L-lactate in the muscle, metabolically exhausted muscle with low AEC and activated PO activity in the haemolymph).
- However, post-discard survival for prawns infected with Hematodium spp. was lower (more than 50% were moribund or dead after 48 h).
- Similarly, damaged animals showed a lower post-discard survival than non-damaged ones indicating that damage is an important survivability factor (again for damaged animals 50% were moribund or dead after 48 h).
- Although absolute survival was not affected by higher air temperatures recorded in June, data from vigour index and physiological measures indicated that animals were more physiologically compromised in June than in March.
- Behavioural trials where post-discard Nephrops were observed at 30 m depth showed that animals subjected to air-exposure for < 60 mins recovered rapidly on re-immersion. They began to move around on the seabed within 2 minutes and appeared to exhibit normal behaviour in relation to interactions with each other.
- However, prawns exposed to air for around 90 mins took up to 10 mins to recover on the seabed. This time was sufficient to attract other scavengers and predators, such as crabs, which were observed attacking recovering Nephrops. At least in one instance escape attempts by the Nephrops were unsuccessful.
- The behaviour trials showed that discarded Nephrops, which are not physically damaged or exposed to long periods of air-exposure, recover rapidly on re-immersion and exhibit ‘normal’ movement and escape behaviours.
- Modelling the probability of death: If we were to consider damaged and infected animals as non-survivors, a stepwise regression was conducted in order to estimate the probability of death (95% confidence interval). Following this approach the probability of dying was around 29% in winter/early spring and 15% in early summer.
- Following this conservative approach if the fishermen were to land damaged discards the survival of discarded un-damaged animals was estimated at around 81% in February, and 78% in March and 94% in June (95% confidence interval). This strategy would improve the probability of survival even using this conservative approach.
- This work was carried out in a trawl commercial vessel that mainly targets the live ‘Nephrops’ market, characterised by relatively short tow duration and therefore, data obtained in this project cannot be extrapolated to the whole Nephrops fleet but to similar vessels operating in the west coast and currently targeting the live market.