One reason sharks may be undergoing severe decline is because of a sexual line in the sea, according to research published in the most recent issue of the journal Biology Letters.
The study by Gonzalo Mucientes and coauthors demonstrates marked sexual segregation across the South Pacific Ocean in the world's fastest-swimming shark, the shortfin mako (Isurus oxyrinchus), which in turn has implications for the fisheries effects on shark populations.
The authors studied the shortfin mako and blue (Prionace glauca) sharks caught by a commercial fishing vessel in the southeastern Pacific Ocean over a period of three months, recording capture locations and sexing the fish caught.
The authors also examined capture data for the two shark species in the Pacific Ocean over the last 54 years.
Within the study area, the authors found the sharks to segregate sexually over the study period, with the male sharks being found predominantly in the west and the females in the east.
The cause of this segregation is not clear, with the authors ruling out prey, sea surface temperatures or primary productivity as being possible factors.
The authors surmise that female shortfin mako may be avoiding the males to avoid severe sexual harassment from the males; male shortfin makos are highly aggressive, even during breeding (males often inflict serious bite wounds on females during this time).
The authors contend that because of this sexual segregation, region-specific fishing activities may have disproportionate effects on different components of shark populations.
Such exploitation of sharks exhibiting seasonal sexual segregation could be a major contributor to population declines.
The authors also found evidence for sexual segregation of blue shark in the southeast Pacific region.
Since mature males dominated the catches encountered, this suggested that segregation occurs at a larger spatial scale than the area studied by the authors.
For more information, see the paper: Mucientes, GR, N Queiroz, LL Sousa, P Tarroso and DW Sims (2009) Sexual segregation of pelagic sharks and the potential threat from fisheries. Biology Letters 5, pp. 156"159.