Short-term stress and mortality of juvenile school prawns, Metapenaeus macleayi, discarded from seines and trawls

Abstract  Two experiments were carried out to assess the short-term impacts to juvenile school prawns, Metapenaeus macleayi (Haswell) discarded from commercial seining and trawling in New South Wales, Australia, and the potential utility of modifications to operational and/or onboard handling practices designed to improve survival. For both gears, discards were handled according to two general categories of treatment (termed mild and extreme) chosen to represent the plausible limits of severity during existing commercial operations. A total of 600 trawled and 480 seined school prawns (from both treatments), along with appropriate numbers of controls (previously collected and housed in acclimation tanks), were placed into cages (in groups of 10). Subsets of each of these groups were then destructively sampled over periods of up to 5 days and examined for their mortality and physiological response (measured as levels of l -lactate in the haemolymph). In both experiments, the temporal mortalities in the treatment groups ranged between 0% and 15% and, for the most part, were not significantly different to controls. Prawns in the treatment groups had similar, significantly elevated levels of l -lactate approximately 40 min after being caught, but these returned to levels approaching baseline estimates within 48 h. Under the conditions examined, juvenile school prawns appear capable of withstanding a range of stressors associated with being discarded from active gears, and their fate is not greatly influenced by the duration of gear deployment and subsequent air exposure. However, further work is required to quantify other sources of unaccounted fishing mortality and, ultimately, the impacts that discarding has on stocks.     

Optimising codend configuration in a multispecies demersal trawl fishery

Abstract The relative efficiencies and selectivities of conventional and modified codends were examined in a demersal trawl fishery targeting several species, including eastern king prawns, Penaeus plebejus (Hess), whiting, Sillago spp. and cephalopods. The modifications to codends included: (i) reducing the circumference; (ii) increasing the mesh size in conventional diamond‐mesh designs; and (iii) orientating meshes on the bar so that they were square shaped. The codends were tested against a fine‐meshed control in paired comparisons onboard three commercial trawlers. The conventional codend comprised 41‐mm diamond‐shaped mesh attached to an anterior extension section at a ratio of 150 to 100 meshes and was demonstrated to be non‐selective for the targeted species. Reducing codend circumference to 100 meshes and increasing the size of mesh to 45 mm both improved selection for eastern king prawns, but the lateral mesh openings were estimated to be insufficient to allow juveniles of the other key species to escape. By contrast, codends made from 35‐ and 41‐mm mesh hung on the bar improved the size selection for eastern king prawns and selected stout whiting, Sillago robusta (Stead) (the smallest commercial‐sized fish) across narrow selection ranges and at 50% sizes of retention (L₅₀s) that were closely correlated to the transverse morphology of fish and the maximum mesh opening. With the exception of a reduction in catches of octopus, Octopus spp., by the 41‐square codend, there were no other impacts on commercial catches by the square‐mesh designs. It was concluded that diamond‐mesh codends are inappropriate for use throughout this multispecies fishery and that a modified design comprising at least 35‐mm mesh hung on the bar is required to minimise the fishing mortality of unwanted sizes of the key target species. The results are discussed in terms of the utility of these types of modifications for closely regulating selection in penaeid‐shrimp trawls.     

Mortality induced by drifting longline hooks and branchlines in loggerhead sea turtles,estimated through observation in captivity

• 1. Drifting longlines are considered a major threat to endangered sea turtle populations worldwide. However, for a number of reasons, the mortality rate of captured turtles is not known with any certainty.
• 2. Information on 409 loggerhead turtles (Caretta caretta), collected during the day‐to‐day activities of a turtle rescue centre in Lampedusa island, central Mediterranean, in the period 2001–2005 has been analysed.
• 3. Observations indicate that: (i) drifting longlines are a major cause of mortality for sea turtles in the area; (ii) in addition to the hook, the piece of line attached to it (branchline) can easily cause death if it is long enough and well‐anchored; (iii) hooks and branchlines cause death in the short and long term, respectively; (iv) a turtle with a hook in the lower oesophagus/stomach has a very low chance of surviving the combined effect of hook and branchline; (v) the mortality of turtles with a hook in the mouth or higher oesophagus is probably important, though less than that of turtles with a hook in the lower oesophagus/stomach; (vi) in the study fishery, the average mortality of a turtle caught by a drifting longline is probably much higher than 30%.
• 4. Without specific investigations on the mortality of turtles with hooks in the mouth or higher oeasophagus, which are usually removed, the mortality induced by drifting longlines will remain unknown, preventing a full understanding of the effect on population growth and the real effectiveness of conservation measures such as use of different hooks and fishing depths, and proposals for adequate fishery management measures.
• 5. The number of turtles captured by drifting longlines should be drastically reduced, and because of the above uncertainty and the socio‐economic importance of the fishery sector, an ecosystem‐based management scheme should be promoted that is not limited to addressing only the turtle issue.

Copyright © 2007 John Wiley & Sons, Ltd.     

Post‐release mortality estimates of loggerhead sea turtles (Caretta caretta) caught in pelagic longline fisheries based on satellite data and hooking location

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Depredation by harbor seal and spiny dogfish in a Georges Bank gillnet fishery

Abstract Depredation can have economic, social and ecological impacts by decreasing fishery landings and threatening marine predator populations through increased risk of entanglement. This investigation quantified the loss of landings attributed to harbour seal, Phoca vitulina L., and spiny dogfish, Squalus acanthias L., depredation in the Georges Bank, Massachusetts, USA gillnet fishery. Approximately 1361 kg (2.38%) of fish was discarded from a total possible catch weight of nearly 57 300 kg because it was unsalable following depredation. Spiny dogfish accounted for 1134 kg (1.98%), and harbour seal accounted for 227 kg (0.40%) of discards. This represented an estimated US$2250 loss in profits from a total possible catch value of almost US$61 800, signifying a 3.64% market‐value loss during the assessment period. These results suggest that discarding due to harbour seal and spiny dogfish depredation results in a relatively small market loss to fishers, although additional information and research is needed before this can be confirmed. This study is the first on this subject in the fishery and provides baseline information on depredation rates that could be useful if harbour seal and spiny dogfish populations continue to increase.     

Annual survival probabilities of juvenile loggerhead sea turtles indicate high anthropogenic impact on Mediterranean populations

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Identification and evaluation of shark bycatch in Georgia’s commercial shrimp trawl fishery with implications for management

Abstract Many US states have recreational and commercial fisheries that occur in nursery areas occupied by subadult sharks and can potentially affect their survival. Georgia is one of few US states without a directed commercial shark fishery, but the state has a large, nearshore penaeid shrimp trawl fishery in which small sharks occur as bycatch. During our 1995–1998 investigation of bycatch in fishery‐dependent sampling events, 34% of 127 trawls contained sharks. This bycatch totalled 217 individuals from six species, with Atlantic sharpnose shark, Rhizoprionodon terraenovae (Richardson), the most common and finetooth shark, Carcharhinus isodon (Müller & Henle) and spinner shark, Carcharhinus brevipinna (Müller & Henle), the least common. The highest catch rates for sharks occurred during June and July and coincided with the peak months of the pupping season for many species. Trawl tow speed and tow time did not significantly influence catch rates for shark species. Gear configurations [net type, turtle excluder device (TED), bycatch reduction device] affected catch rates for shark species. Results of this study indicate gear restrictions, a delayed season opening, or reduced bar spacing on TEDs may reduce shark bycatch in this fishery.     

Square‐mesh codend length and selectivity in southeastern Australian stow nets

Abstract The relationship between the amount of square mesh in codends and selectivity was investigated for an Australian penaeid stow‐net fishery. Three lengths (3, 2 and 1 m) of square‐mesh codend made from 27‐mm mesh hung on the bar were alternately tested with a conventional 34‐mm diamond‐mesh design during two covered‐codend experiments. Compared with the conventional codend, the square‐mesh configurations incrementally selected school prawns, Metapenaeus macleayi (Haswell) across narrower selection ranges (SR) and mostly at greater sizes at 50% retention (L₅₀), while retaining fewer fish. Irrespective of the mesh configuration or square‐mesh codend length, there were significant differences between experiments (attributed to water flow) and impacts of catch weight on the selectivity of school prawns. Both variables had a negative relationship with L₅₀, while water flow similarly affected SR. This study reaffirms the utility of square‐mesh codends as a key input control for managing the selectivity of penaeid‐catching gears.