However, all crabs that were at large for more than a week made migrations to depths shallower than 15 m. Crabs spent the majority of time in conditions that did not present a physiological challenge and at depths below 15 m. In the field, crabs were fitted with archival CTD data tags to monitor the salinity, temperature and depth experienced at 10 min intervals. The results indicate that the development of potting methods for New Zealand scampi has the potential to reduce bycatch of some fish species, but will need to focus on reducing hagfish and invertebrate bycatch through improved pot design and spatially-targeted fishing.Ĭombined field and laboratory experiments were carried out to investigate the salinity and thermal preference of adult Dungeness crabs, Cancer magister (Dana). The bycatch from the pots included proportionately more invertebrates and hagfish, and less other fish species when compared to previous studies analysing the bycatch of the trawl fishery for New Zealand scampi. Additionally, both hagfish and total bycatch showed significant spatial variation at two scales among individual deployments of strings of pots within locations, and among locations within the sites. The pot design with the highest levels of bycatch at both sites was Pot 2, a two-chambered parlour pot which caught between 1.5–10.1 times more total bycatch than the other pot designs. The different pot designs were observed to significantly affect the bycatch of hagfish ( Eptatretus cirrhatus) and total bycatch at both sites, while at the Chatham Rise site the bycatch of fish species and invertebrate was also affected by pot design. The effects of bait and location on bycatch were also assessed by using three different bait types and fishing the pots at two different scampi commercial fishing sites around 200 km apart. This hypothesis was assessed by comparing the bycatch rates from four different designs of pots used in similar deep sea lobster fisheries overseas. Pots have been suggested as an alternative fishing method that would reduce the high levels of fish bycatch in the New Zealand scampi ( Metanephrops challengeri) trawl fishery. Pots (also known as creels or traps) are used as a less impactful alternative to trawling in a number of crustacean fisheries, although bycatch may still occur. We conclude that taxonomic relatedness to bait, degree of carcass damage and hunger of the scavenger all interact in determining foraging decision-making even by so-called generalist scavengers.īottom trawling is a fishing method that typically produces high levels of bycatch and is associated with extensive benthic habitat damage. The localized presence of odours emanating from dead conspecifics did not cause crabs to remain inactive in shelter. Conversely, crabs of the fed batch were significantly more reluctant to enter creels containing damaged conspecifics. Starved crabs remained attracted to fish bait, despite the proximity of dead conspecifics. The effects of hunger on these responses were tested in a laboratory experiment in which the responses of starved and fed batches of C. The magnitude of th response was related strongly to the extent to which C. The remaining crustacean taxa (of the above) were unaffected by this treatment. maenas significantly reduced the capture of conspecifics, but significantly enhanced the capture of the taxonomically unrelated species A. The addition of substantially fractured C. Capture rates of these species by creels baited with dead fish plus variously damaged C. Field studies using fish-baited creels have confirmed that Carcinus maenas, Necora puber and Pagurus bernhardus (Crustacea: Decapoda), Buccinum undatum (Gastropoda) and Asterias rubens (Echinodermata) are prominent scavenging species in shallow waters in the Clyde Sea area.
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