Population Dynamics of the Porbeagle in the Northwest Atlantic Ocean

Abstract

A virgin population of porbeagles Lamna nasus in the northwest Atlantic Ocean supported annual catches of up to 9,000 metric tons (mt) in the early 1960s before the fishery collapsed in 1967. Low and apparently sustainable catches of about 350 mt in the 1970s and 1980s allowed the stock to partially rebuild before a new fishery arose in the early 1990s. The response of the population to this renewed fishing pressure has been unclear until now. However, a new population dynamics analysis suggests that population abundance has once again declined. On the basis of more than 140,000 length measurements, an extensive catch rate index, a confirmed growth model, and a catch‐at‐age matrix, it appears that at least 90% of the sexually mature population has been lost as fishing mortality has increased. Independent measures of fishing mortality (F) based on Petersen analysis of tag‐recaptures, Paloheimo Zs, and a population model all suggest that fishing mortality was about 0.20 in 2000. Biological reference points based on life table analysis indicate that fishing at F0.1 = 0.18 will result in population collapse, that F = 0.08 corresponds to zero population growth, and that fishing mortality at maximum sustainable yield is about 0.04. Porbeagles have a low pup production rate and mature considerably after the age at which they first appear in the fishery. In light of the very low numbers of mature females now found in the population, it is unlikely that even the strict quota management now in place will allow the population to rebuild quickly. However, the shark fishing industry has actively supported scientific research and conservation practices in recent years, suggesting that long‐term sustainability may still be possible.

Changes in baseline growth and maturation parameters of Northwest Atlantic porbeagle, Lamna nasus, following heavy exploitation

“…We believe that this apparent decrease in relative growth following exploitation is attributable to sampling bias. The age at recruitment to the porbeagle fishery declined substantially after 1993, from 6-7 years to 2-3 years (Campana et al 2002a). Smaller, slower-growing individuals in the youngest age classes would have been less vulnerable to capture by the longline gear during the earlier sampling period and, therefore, were most likely underrepresented in our historical sample.…”

“…Historically, exploitation was most intense during the first few years of the fishery. From 1961 to 1964, total reported annual landings in the Northwest Atlantic rose from 1900 to 9000 tonnes (t) (Campana et al 2002a). These intense fishing efforts precipitated the collapse of the Norwegian fishery in the late 1960s and reduced the population to an estimated 50% of the virgin abundance (DFO 2005).…”

“…These intense fishing efforts precipitated the collapse of the Norwegian fishery in the late 1960s and reduced the population to an estimated 50% of the virgin abundance (DFO 2005). A modest recovery of porbeagle numbers occurred throughout the 1970s and 1980s, as landings averaged less than 350 t·year -1 (Campana et al 2002a). However, annual catches increased to 1000-2000 t during the 1990s, and the abundance of porbeagles once again fell sharply (Campana et al 2003).…”

The Biology and Ecology of the Porbeagle Shark, Lamna Nasus

“…Porbeagles have proven to be vulnerable to overfi shing in the Northwest Atlantic: A target longline fi shery in the 1960s lasted only 6 years before collapsing. This fi shery was revived by Canadian and US vessels in the 1990s, but catch levels during the last decade appear to be unsustainable (Campana et al, 2002a(Campana et al, , 2008. In the Southern Hemisphere, porbeagles have not been targeted, but they are frequently taken as bycatch in tuna longline fi sheries (Francis et al, 2001;Ayers et al, 2004).…”

Evaluation of two underwater biopsy probes for in situ collection of shark tissue samples

“…Many exploited shark populations are currently in decline (Campana et al 2002, Baum et al 2003 or low in abundance (Graham et al 2001, Baum & Myers 2004. Shark species often have limited resilience to exploitation due to their life history traits of late age at maturity and low fecundities (Holden 1974).…”