Postrelease survival, movements and thermal habitats of pelagic sharks

Postrelease survival, vertical and horizontal movements, and thermal habitats
of five species of pelagic sharks in the central Pacific Ocean

Musyl et al.


From 2001 to 2006, 71 pop-up satellite archival tags (PSATs) were deployed on five species of
pelagic shark (blue shark [Prionace glauca]; shortfin mako [Isurus oxyrinchus]; silky shark [Carcharhinus
falciformis]; oceanic whitetip shark [C. longimanus]; and bigeye thresher [Alopias superciliosus]) in the central
Pacific Ocean to determine species-specific movement patterns and survivalrates after release from longline
fishing gear. Only a single postrelease mortality could be unequivocally documented: a male blue shark which
succumbed seven days after release. Meta-analysis of published reports and the current study (n=78 reporting
PSATs) indicated that the summary effect of postrelease mortality for blue sharks was 15% (95% CI, 8.5–25.1%)
and suggested that catch-and-release in longline fisheries can be a viable management tool to protect parental
biomass in shark populations. Pelagic sharks displayed species-specific depth and temperature ranges,
although with significant individual temporal and spatial variability in vertical movement patterns, which
were also punctuated by stochastic events (e.g., El Niño-Southern Oscillation). Pelagic species can be separated
into three broad groups based on daytime temperature preferences by using the unweighted pair-group
method with arithmetic averaging clustering on a Kolmogorov-Smirnov Dmax distance matrix: 1) epipelagic
species (silky and oceanic whitetip sharks), which spent >95% of their time at temperatures within 2°C of
sea surface temperature; 2) mesopelagic-I species (blue sharks and shortfin makos, which spent 95% of
their time at temperatures from 9.7° to 26.9°C and from 9.4° to 25.0°C, respectively; and 3) mesopelagic-II
species (bigeye threshers), which spent 95% of their time at temperatures from 6.7° to 21.2°C. Distinct
thermal niche partitioning based on body size and latitude was also evident within epipelagic species.

Fishery Bulletin 109(4):341–368 (2011)

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