the northwestern Pacific pelagic thresher stock has been reduced
34.3% over the past 20 years and that this stock is overexploited.
In addition, the global population has been significantly decreasing
and the species has been placed in the IUCN red list as vulnerable
[46]. Approximately 23% of the tissue samples used in this study is
identified as pelagic thresher shark, which could indicate that this
species is under high fishing pressure in Taiwan. According to
[45], this species is extremely vulnerable to overexploitation and is
sensitive at juvenile and adult stages. Therefore, the closure of
nursery grounds or the institution of a size limit is urgently needed
to ensure the sustainable utilization of the stock. The silky shark
(Carcharhinus falciformis) is valuable to a wide variety of pelagic
fisheries and is taken in large numbers, but there are no population
estimates and most catches are unreported. Silky sharks rank
among the three most important sharks in the global shark fin
trade [47], with 0.5–1.5 million traded annually. In Taiwanese
waters, the average annual landing (whole weight) of sharks at
Nanfangao was 5,669 MT in 2001–2010. Reported silky shark
landings only composed 1.04% of the total shark landings [44];
however, approximately 23% of our tissue samples were from the
silky shark. This suggests an increased level of silky shark
exploitation in the past few years, a contribution of fish landings
from other harbors, or contributions by un-reported landings.
However, it is difficult to determine the reason for this discrepancy
due to the lack of species-specific landing data. According to [48],
the global silky shark population is decreasing and Carcharhinus
falciformis has therefore been placed on the IUCN red list under the
criteria of near threatened and could meet the criteria for VU in
the future.
The shortfin mako (Isurus oxyrinchus) is an important species for
pelagic longline, drifting, and set gill nets, and for hook-and-line
fisheries in the eastern Pacific [49]. An instantaneous rate of
decline of 38% between 1992 and 2005 in the Northwest Atlantic