Clear population structuring in large, open-ocean sharks at this scale is striking and has not been previously reported. The sexual ‘line in the sea’ we observed between male and female shortfin mako is intriguing because this species is the world's fastest swimming shark (clocking speeds up to approx. 70
) and capable of long-distance movements. Tagging shows trans-Atlantic migrations are rare however (Casey & Kohler 1992
), but with sufficient genetic exchange among stocks for a single species worldwide (Heist 2008
). Nonetheless, this exploited shark displays pronounced sex and size segregation at the regional scale, which does not appear to closely reflect prey, SST or primary productivity, at least over the time scale of this study. Furthermore, the observed pattern appeared to be principally the result of spatial rather than temporal effects because the large changes in sex ratio were too abrupt (occurring over 8 and 24 days) within the core summer months to be consistent with large-scale, synchronous movements of the sexes as the survey progressed, which would be expected if segregation were wholly temporally driven (see Discussion in the electronic supplementary material).
Numerous hypotheses have been proposed to explain sexual segregation in animals, but how and why it occurs remains controversial and largely unresolved for many taxa (Wearmouth & Sims 2008
). For the sexually size dimorphic scalloped hammerhead shark Sphyrna lewini
(females grow larger), Klimley (1987)
proposed that females segregated from males by moving to offshore habitat to feed on different, more energy-rich prey that conferred increased growth rates, such that maturity was reached at a larger body size than similar aged males; a larger female body size is necessary to support large, well-developed embryos. It was suggested that this strategy would act to match the reproductive lifetime of females with that of males within the same cohort. Shortfin mako exhibit sexual body size dimorphism with females growing up to 4
m in length, some 30 per cent larger than males, and giving birth to a few large young (embryo at-birth length, approx. 0.7
m; frequency, 4–16 per female; Compagno 2002
). Although the present study could not conclusively identify behavioural mechanisms, our results indicate that mako sex segregation is probably unrelated to different nutritional requirements, because male and female diets were not different and basal productivity between male and female habitats was a poor predictor of the observed pattern.
Sexual segregation in mako shark in this study was observable at the large spatial scale but was not absolute because some mixing was evident. Males and females were captured on the same longline sets at a number of locations, principally around the line separating western from eastern sectors, but also along the thermal front boundary zone in the south (a
). Thermal fronts are often prey rich and act to aggregate predators with greater apparent mixing of the sexes owing to feeding or courtship opportunities (Sims et al. 2000
). Although we are unable to provide an explanation for why shortfin mako segregate sexually, it is possible that it occurs owing to social factors. Courtship and mating in sharks are highly aggressive during which (often multiple) males inflict serious bite wounds on females (Stevens 1974
). It is possible that mako shark sexual harassment (by males) results in fitness consequences for females (Magurran & Seghers 1994
), interactions that manifest at the large geographical scale as sexual segregation. That mature females were not captured in large numbers (n
=15) suggests that they were absent from the study area, which may also reflect avoidance behaviour.
The finding of marked sexual segregation in a fast-swimming, highly mobile pelagic shark at the broad scale has implications for assessing fisheries effects on shark populations. Complex structuring coupled with region-specific fishing activities may have disproportionate effects on different components of shark populations. In support of this, we found sex differences in potential exposure to fishing effort for I. oxyrinchus
owing to geographical separation of the sexes (see Discussion in the electronic supplementary material). For shortfin mako in this study, we hypothesize that more intense longlining in the west, if it occurs over the shorter, seasonal term, has the potential for higher relative catch rates of males but lower catches of females. Exploitation of sharks exhibiting seasonal sexual segregation could be a major contributor to population declines. For example, the seasonal capture of sex-specific schools of mature female spurdog Squalus acanthias
in the English Channel may have resulted in stock collapse in just a few years (Ford 1921
We also found evidence for sexual segregation of blue shark in the southeast Pacific region, since catches were dominated by mature males, suggesting that segregation occurs at a larger spatial scale than the area studied here. Our findings are consistent with tagging and surveys showing that blue shark sexually segregated over very large, perhaps even ocean-basin scales (Stevens 1990
). Nevertheless, even with sexual structuring at these scales, blue shark populations may also be affected by sex differential exploitation. For example, P. glauca
in the western Atlantic are thought to segregate sexually and our proposal of sex-biased exploitation seems supported because male relative abundance declined by 80 per cent between 1977 and 1994 but no change was discernible for females over the same period (Simpfendorfer et al. 2002
What these and the current study indicate is the need for wide-scale, spatially referenced recording of shark sexes by global high-seas fisheries. However, given the lack of even the most basic shark catch data for most fisheries, the potential problem we highlight may already have impacted populations. There is an urgent need for proper reporting by high-seas fisheries of shark catches by species, number of individuals and biomass, together with their sex.