Project 4.8.4

Shark life histories and their implications for Great Barrier Reef fisheries management

Project Leader: Dr Andrew Tobin, JCU

This project has made significant steps towards addressing critical knowledge gaps about important life history traits of nine shark and ray species that interact with the East Coast Inshore Finfish Fishery (ECIFF) within the Great Barrier Reef World Heritage Area. The major findings were:

  • Maximum age and length demonstrates varied species-specific biological profiles. Extremes are represented by the small and short-lived milk shark and the large and long-lived great hammerhead shark, with maximum length and age recorded at 930 mm and 4.5 years and 4400 mm and 22 years respectively. These extremes are also represented in the growth rates of these sharks, with male milk sharks exhibiting the most rapid growth (k = 0.94) and great hammerheads the slowest (k = 0.04 and 0.085 for female and male sharks respectively).
  • Australian blacktip and spot tail sharks, which dominate the elasmobranch harvest within the ECIFF, display medium maximum sizes (1800 mm and 1310 mm respectively) and longevities (13.9 and 13.6 years respectively).
  • Modeled von Bertalanffy growth profiles further reinforce the variation between the age, growth and longevity characteristics of individual species. The smallest and youngest species, the milk shark, grows quickly towards a relatively sharp asymptote; the medium sized Australian blacktip and spot tail sharks grow less quickly with stronger asymptotic tendencies for the smaller spot tail than for the Australian blacktip; while scalloped and great hammerhead sharks have a slower overall growth rate that shows very little tendency to reach an asymptote.
  • Male Australian blacktip, spot tail, milk and great hammerhead sharks tend to grow faster, but females tend to have greater longevity and larger maximum lengths than males.
  • Male milk, spot tail, Australian blacktip and great hammerhead sharks mature at smaller lengths than their female partners. Only the milk and spot tail sharks matured at lengths less than 1 m, while great hammerhead sharks exhibited the longest length-at-50% maturity at 2284 mm and 2258 mm for females and males respectively.
  • Pupping season was the most consistent life history trait between species. All species investigated pup during the warm summer months, with the exception of milk and great hammerhead sharks, which pup aseasonally.

Some of the species (such as milk sharks and spot tail sharks) display life history traits that confer a relatively high resilience to fishing. The primary shark species harvested within the ECIFFF is the Australian blacktip shark, and, based on the new information generated by this project, this species is likely to be less resilient to fishing impacts (relative to milk and spot tail sharks) owing to its slower growth rate, longer lifespan and larger size at maturity. The scalloped and great hammerhead sharks display a mix of biological traits, with some conferring resilience while others suggest vulnerability. Large maximum size and large length at maturity suggest low resilience while very large litter sizes (n > 20) is a resilient character. This new information has been incorporated into an ecological risk assessment for inshore species that are targeted by commercial and recreational fishing.


Impacts of the rezoning of the Great Barrier Reef Marine Park on commercial inshore net fishers

Project Leader: Dr Andrew Tobin, JCU

The impacts of the July 2004 rezoning of the Great Barrier Reef Marine Park on patterns of fishing effort and catch within the inshore commercial net fishery were examined through the analysis of commercial logbook data recorded between 1988 and 2008. Over the period examined, the annual landings within this fishery have increased by over 600%, with 360 t of shark and finfish product harvested in 1988 compared with 2210 t of shark and finfish product landed in 2008. The largest annual catch of 2708 t was recorded in 2003, driven largely by increased activity (effort and catch) within the inshore shark fishery, with slight increases also being recorded for barramundi and threadfins. The period 2000 to 2008 was distinctly different from previous years, showing strong similarity for the targeting (days fished) and harvesting (kg landed) of tropical inshore shark. Subsequent analyses compared the catch landed and days (fi shed) during the four years prior to the rezoning and the four years since, for each of the five main target species of the fishery (namely shark, grey mackerel, barramundi, threadfin salmon and garfish) for the whole Great Barrier Reef World Heritage Area, and within each of the five sub-regions (Far North, Cairns, Townsville, Mackay and Capricorn). Results showed that:

  • The effort (days fished) within the fishery tends to increase post-rezoning, while catches show slight decreases, although neither trend was statistically significant. Only shark showed a significant decline in landed catch, while grey mackerel showed a significant increase in effort that is likely to be attributable to improvements in gear that allows fishing even in poor weather.
  • Within sub-regions, no clear differences between pre- and post-rezoning were evident in either catch or effort of commercial net fishers in the Far North, Cairns or Townsville regions. In the Mackay region, there was no clear difference in effort between pre- and post-rezoning, but there was a difference in catch composition attributable to a decrease in shark catches post-rezoning. The Capricorn region showed an increased presence of shark in the catches post-rezoning, which corresponded to increased effort targeting shark post-rezoning.
  • Overall, only slight changes in effort and targeting behaviours of the commercial fishing fleet were detectable, particularly for shark and grey mackerel.

The commercial inshore net fishery appears to have considerable adaptive capacity that allows individual fishers to catch a range of potential target species depending on seasonal presence, market price, weather, and participation in other fisheries (most commonly crab and line).