A Comprehensive Review of the Biology and Preliminary Investigation of Interactions with the U.S. Pelagic Longline Fishery for the Shortfin (Isurus oxyrinchus) and Longfin (Isurus paucus) Mako Sharks

Defense Date

6-2007

Document Type

Capstone

Degree Name

M.S. Marine Biology

First Advisor

David Kerstetter

Second Advisor

Richard E. Spieler

Abstract

The shark genus Isurus is cosmopolitan and represented by two species: the shortfin mako Isurus oxyrinchus and the longfin mako lsurus paucus. Besides a few cetaceans (Visser et al., 2000) and man, the adult mako shark is not prey to any marine animal, but is considered an apex predator at the top of the marine environment food web. Shortfin and longfin mako sharks are often captured incidentally in various pelagic fisheries throughout the world and may be vulnerable to overfishing (Baum et al., 2003; ICCA T, 2004). Researchers have documented such declines and many shark populations in general at various geographical locations are believed to be at critical population levels or may have already collapsed (Baum et al., 2003; Baum and Myers, 2004; Baum and Myers, 2005) due to direct or indirect fishing activities (Martin, 2005). Sharks are long lived, slow growing, reach sexual maturity late in their life history, and produce few offspring; thus, overall population growth is slower and sharks have a lower growth rate than most teleosts (Musick, 1999). According to the JUCN World Conservation Union's Red List of Threatened and Endangered Species, the shortfin mako shark was assessed and listed under the category Lower Risk (Near Threatened (NT)) in 2002. Taxa under this category do not qualify for Conservation Dependent status, but are close to qualifying under the Vulnerable category. In 2005, the longfin make shark was assessed and listed in the Vulnerable category under the same Red List of Threatened and Endangered Species. More recently, the Committee on the Status of the Endangered Wildlife in Canada (COSEWIC) drafted an assessment and population status for the shortfin mako shark (COSEWIC, 2006). Based on the review, the committee decided to list the species as Threatened under the criteria A2b. According to the COSEWIC definitions, Threatened is defined as a species likely to become endangered if limiting factors are not reversed. Presently, little to no information is available for the longfin mako shark and limited information is available for the shortfin mako shark. Although COSEWIC (2006), ICCA T (2004), and IUCN (2002) have conducted status reviews on the shortfin mako shark, this report provides the first and only complete comprehensive review that has used all available information on the shortfin and longfin mako shark.

This comprehensive review was primarily based on published literature obtained by using the Nova Southeastern University library online database search system and the National Marine Fisheries Service (NMFS) Pelagic Observer Program (POP) database.

The shortfin mako shark is an apex predator that has some special evolutionary traits unlike any other shark species. There are only five species classified in the Lamnidae family (shortfin mako, longfin mako, porbeagle, salmon, and white shark). Review of molecular genetics studies revealed that only one species of the shortfin mako shark exist worldwide (Heist et al., 1996). Distinct from most sharks, the shortfin mako shark has the ability to regulate its internal body temperature and closely resembles the physiology of some tunas. Distribution information has revealed the shortfin and longfin mako shark are highly migratory species found in every major ocean. Both species are captured in commercial fisheries (pelagic longlines and gillnets) worldwide and abundance trends show that the shortfin mako shark has declined over the last 30 years. The shortfin mako shark is captured in great numbers and ranks as one of the most dominant species caught in pelagic longlines and gillnets. The longfin mako, although also caught in similar fishing gears, is rare in catch composition data. Today, worldwide size composition data shows that male shortfin mako sharks are being caught at or below the size at maturity and below the female shortfin mako shark size at maturity. Moreover, size composition information shows that commercial fisheries are exploiting virtually the species entire size range (60-340 em FL). Recent age and growth information studies indicate that the shortfin mako shark is one of the slowest growing and longest lived extant shark species. Reproduction information suggests that the shortfin mako shark has one of the longest gestation periods and reproductive cycles within the elasmobranchs, More importantly, intrauterine cannibalism has recently been documented in the shortfin mako shark, which has only been previously reported in the sand tiger shark Carcharius taurus. Unlike most shark species, the shortfin mako shark is economically important for both its quality of meat and fins. Even though the species is physiologically unique, economically important, and exploited in significant relatively large numbers, this review has revealed that there are limited specific regulations and management plans currently in place, and recovery plans for the shortfin mako shark are lacking.

Based on the results of this review, it is highly recommended that further research in age specific population dynamics, fishery yield models, and specific domestic and international management measures be implemented for the conservation and sustainability of the longfin and shortfin mako sharks.

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