Pelagic Shark Studies Advance Understanding of Ways to Reduce Bycatch
One of the important information gaps in management of pelagic fisheries concerns the mortality of animals caught in the fishing gear and then put back in the sea by fishermen. The animals released are called "bycatch". Strategies for mitigating bycatch in large-scale commercial fisheries require an understanding of the post-release survival of bony fishes, sharks and sea turtles as well as information about the habitats and movement patterns of these animals. Large pelagic sharks are not generally targeted by fishermen, but make up the majority of the bycatch in pelagic gill nets and longline fisheries that target swordfish (Xiphias gladius); blue shark (Prionace glauca) is particularly common in the bycatch.
To learn more about bycatch mortality, Mike Musyl, a scientist at PIFSC employed by the University of Hawaii Joint Institute for Marine and Atmospheric Research, joined with several colleagues to deploy 71 pop-up satellite archival tags (PSATs) on the five most commonly caught species of pelagic shark in the Hawaii-based commercial longline fishery. These are blue shark; shortfin mako shark, Isurus oxyrinchus; silky shark, Carcharhinus falciformes; oceanic whitetip shark, C. longimanus; and bigeye thresher shark, Alopias superciliosus. The objective of the research was to determine species-specific horizontal and vertical movement patterns and survival after release from longline fishing gear.
All five species studied have life-history characteristics that make their populations more vulnerable to over-exploitation than the stocks of tunas and billfishes typically targeted by longline fisheries. Moreover, there is little or no information about their movement patterns and habitats. In an analysis of the archival tagging data, only a single post-release mortality could be unequivocally documented: a male blue shark which succumbed seven days post-release. The depth and temperature data collected by the tag suggested that the shark died from injuries it sustained during capture and handling, rather than from predation. Meta-analysis of blue shark mortality data from published and ongoing research (n = 78 reporting PSATs) indicated that the overall post-release mortality from longline gear was 15% (95% CI: 9–25%). Favorable rates of post-release survival suggest catch-and-release in longline fisheries can be a viable management tool to protect parental biomass of shark populations, although fishery related factors (hook type, soak time, handling of catch during release) can influence survival rates.
Musyl and his colleagues found that pelagic sharks displayed species-specific depth and temperature ranges, although with significant individual temporal and spatial variability in patterns of vertical movement. Such movements were affected by stochastic events (e.g., El Niño-Southern Oscillation [ENSO]). The researchers separated pelagic sharks, along with some other pelagic species that have been PSAT-tagged (swordfish, bigeye tuna, and marlins) into three broad groups (see figure below) based on daytime temperatures of their occupied habitat. They defined the groups by applying the UPGMA (unweighted pair-group method with arithmetic averaging) clustering algorithm to the Kolmogorov-Smirnov Dmax distance matrix. The resulting groups of sharks, and the temperature ranges they occupied are characterized as: (1) epipelagic species (including silky and oceanic whitetip sharks) which spent > 95% of their time at temperatures within 2°C of the sea surface temperature, (2) mesopelagic-I species (including blue and shortfin mako sharks) which spent 95% of the time at temperatures from 9.7 – 26.9°C and 9.4 – 25.0°C, respectively, and (3) mesopelagic-II species (including bigeye thresher shark) which spent 95% of the time at temperatures from 6.7 – 21.2°C.
The researchers also found evidence of distinct thermal niche partitioning within epipelagic species. They were able to separate silky shark populations by body size and region (individuals occurring either north or south of ~10° N, the latitude which delimits the North Equatorial Countercurrent). For the most part, the topology of clusters did not appear to correlate with ENSO variability, phylogeny, life history characteristics, ecomorphotypes, neural anatomy, relative eye size, physiology, or the presence of regional endothermy. This indicates that other factors (e.g., ontogeny, latitude, locomotion, diet, and dimensionality of the environment) influence the structure and spatial and temporal stability of thermal habitats. The results suggest that habitat structure for the epipelagic silky and oceanic whitetip sharks can be adequately estimated from two dimensions (these species spend most of their time in the warmest available water). By contrast, three dimensions will be required to describe the extended vertical habitat of the species that were classified as mesopelagic-I (blue shark, shortfin mako shark) and mesopelagic-II (bigeye thresher shark).
This research has been accepted for publication in Fishery Bulletin (Musyl MK, Brill RW, Curran DS, McNaughton LM, Kikkawa B, Fragoso N, and Moyes CD. 2011. Post-release survival, vertical movements and thermal niche partitioning in five species of pelagic sharks released from longline fishing gear in the central Pacific Ocean).
