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Experiments were conducted aboard longline vessels operating in the Seychelles targeting swordfish and tuna. Experimental longlines fitted with hook timers and temperature depth recorders were used to identify differences in species composition, quantity and depth of catch between night sets using a shallow basket (swordfish target) and day sets using both shallow and deep baskets (tuna target). The majority of species were caught during day sets (75%). Lancetfish was the main bycatch species during daytime sets. Sharks were the most common nightime bycatch species, with blue and shortfin mako sharks being the predominant shark species. All marketable species were caught in the 150 m layer, marlins were never caught below 100 m. Sharks were caught below 150 m during both night and day sets. For other bycatch species, all were caught in depths less than 150 m during night sets. Daytime depth captures varied.

Bycatch interactions with deep-sea elasmobranchs are increasingly common and can lead to dramatic declines in abundance over short time scales. Sharks hooked in the deep sea could face a higher likelihood of severe physiological disturbance, at-vessel mortality, and post-release mortality (PRM) than their shallower counterparts. Unfortunately, robust PRM rates have not yet been estimated for longline-caught deep-sea sharks, and as such are not currently incorporated into total fishery mortality estimates or bycatch assessments, limiting the effectiveness of current conservation or management initiatives. We empirically estimated PRM for 2 focal taxa of deep-sea shark, the Cuban dogfish Squalus cubensis and the gulper shark Centrophorus sp., using post-release enclosures deployed at-depth. We calculated 24 h PRM rates of 49.7 ± 8.5% (mean ± SE) for S. cubensis and 83 ± 16% for Centrophorus sp. and identified blood lactate, total length, glucose, and vitality scores as predictors of PRM in S. cubensis. We also observed all 24 h PRM within 11 h post-capture and demonstrated the effects of recovery depth and at-vessel blood chemistry metrics on post-release behavior. Our results suggest that PRM rates of deep-sea sharks are high and highlight the need for filling in this gap in fishery mortality estimates for other common discards in the future.

Fishery data collected from the Reunion Island and Sechelles pelagic longline fisheries (targeting swordfish and tuna) between 2004 and 2015 was used to propose depredation indicators and to assess depredation levels in these fisheries. The interaction rate (depredation) was significantly higher for sharks compared to toothed whales in both fisheries. However, there was a higher depredation per unit effort from toothed whales compared to sharks indicating the impact was more significant. The gross depredation rate in the Seychelles fishery was 18.3%. A slight increase in the gross depredation rate was seen over time in the Reunion Islands fishery (4.1% 2007-2010 and 4.4% in 2011-2015). These indicators were used to estimate the economic loss to these fisheries from depredation events. For the Reunion Island fishery the economic loss was 0.09 EUR/hook and 0.86 EUR/hook for the Seychelles. The results suggest a southward decreasing tooth whale and shark depredation gradient. The depredation levels in the Seychelles are some of the highest in the world and may suggest a 'hot spot' of interactions between toothed whales and pelagic longlines.

The study quantified the impact of turtle excluder devices (TEDs) on the catches of various elasmobranchs caught off northern Australia using data collected during a previous study (Brewer, et al., 2006). The effect of fish size and various aspects of TED design such as grid orientation, grid angle, and bar space were quantified to determine their effect on the escape of elasmobranchs from shrimp trawls. The relatively low number of elasmobranchs encountered during sampling resulted in a lack of power to isolate the effects of the various factors tested. Generally, the study showed that TEDs facilitated the escape of large elasmobranchs, including several species of conservation interest (the scalloped hammerhead, Sphyrna lewini and the zebra shark, Stegostoma fasciatum). Bar space and orientation were important TED design factors affecting the escape of elasmobranchs. Top-shooter TEDs enabled more Carcharhiniformes (“ground sharks”, including requiem sharks and hammerhead sharks) to escape penaeid trawls, while bottom-shooter TEDs increased the escape of Myliobatiformes (rays, including stingrays and cownose rays). However, the TED bar space that facilitates maximum escape of elasmobranchs while maintaining catch of the target species is more difficult to quantify given the low catch rate of elasmobranchs in the trawls. 

The authors worked with creel fishers to trial and assess whether sinking rope was practical to fish with in Scottish inshore waters, and therefore to reduce entanglement risk  to minke whales (Balaenoptera acutorostrata), humpback whales (Megaptera novaeangliae), basking sharks (Cetorhinus maximus), and other megafauna. Overall, the fishers reported few problems with the modified gear, and the gear performed well underwater according to Remotely Operated Vehicle (ROV) footage and sensors. Sinking rope may be a simple, viable option to reduce entanglement risk in the Scottish creel fishery.

This study tested the use of an excluder grid with 90mm bar spacing to reduce bycatch of blackmouth catshark (Galeus melastomus) in  demersal trawl fisheries in the Mediterranean. The 90mm excluder grid was not very efficient in reducing blackmouth catshark bycatch because only the larger individuals were excluded, and the target catches of greater forkbeard fish (Phycis blennoides) and (to a lesser extent) Norwegian lobster (N. norvegicus) was reduced. Results of modeling suggest that reducing the grid bar spacing to 70mm would provide a better trade-off of reducing blackmouth catshark bycatch while keeping a high catch rate of greater forkbeard and Norwegian lobster.

Large escape openings located at the sorting grid panel, which could be a beneficial tool to reduce marine megafauna bycatch, such as Greenland shark (Somniosus microcephalus), did not impact northern shrimp (Pandalus borealis) catch rates in comparison to traditional trawl gear. 

Elasmobranch bycatch significantly decreased in bottom-set gillnets in the Gulf of California using orange LEDs, ultraviolet LEDs, green LEDs, and green glow sticks, with orange LEDs being the most effective at reducing bycatch. As a whole, catch of bony fish was not affected by illumination, although responses were taxonomic-specific.  

This study tested whether a zinc/graphite galvanic electric field reduced capture of elasmobranchs in demersal and pelagic longline fisheries in Florida and Massachusetts (United States). The catch per unit effort of demersal sharks in Florida using the zinc/graphite treatment was significantly lower than the controls, with a 62.3-69.5% reduction in sharks caught on the zinc/graphite treatment. However, there was no difference in  demersal shark CPUE between treatments in Massachusetts. Too few sharks were captured in the pelagic longline trials to allow statistical comparison; however, capture of targeted bony fishes was greatest on hooks with the zinc/graphite treatment, suggesting no negative effect on target catch rates.  Additionally, there was an observed difference in efficacy of the zinc/graphite between Carcharhiniform sharks (more effective) and the Squaliform sharks (less effective), which suggests that the response to this deterrent may be species-specific. Further testing is needed to understand the limitations of the gear and efficacy for pelagic species. 

Tests with captive, as well as wild, juvenile sandbar sharks (Carcharhinus plumbeus) were undertaken to determine the effectiveness of electropositive metals in reducing the interactions with longline gear.  Electropositive metals deterred feeding in groups of captive juvenile sharks for a short period of time and altered the swimming patterns of individuals when food motivation was not present.  In the field, electropositive metals placed within 10 cm of the bottom longline hooks reduced the catch of sandbar sharks by around two thirds, compared to the catch on hooks with a placebo.