The biology and fisheries status of seabreams (family: sparidae) in Hong Kong and adjacent waters

Abstract

Sparids (family Sparidae), or seabreams, are economically valuable species in Hong Kong and adjacent waters. Overfishing of this group of fish is suspected due to the intensive unmanaged and unmonitored nature of fisheries in the region. Yet, the biology, fishery history and current status of local Sparidae are largely unknown. Such information is critical for understanding the fishery condition and for developing practical and relevant management recommendations.

The reproductive biology of the seabreams Acanthopagrus schlegelii, Evynnis cardinalis and Pagrus major were investigated. Histological analysis showed E. cardinalis and P. major to be functionally gonochoristic, the latter having a bisexual juvenile stage and considered to be a rudimentary hermaphrodite, and A. schlegelii is a protandric hermaphrodite. Standard lengths (SL) at female maturity for A. schlegelii, E. cardinalis and P. major are 29.1 cm, 11.7 cm and 33.2 cm respectively. Spawning periods of the three species are from November to March. Gonadosomatic index suggested that A. schlegelii and E. cardinalis are pair spawners while P. major may be a group spawner.

Age and growth were studied in the protandric Acanthopagrus schlegelii by reading growth rings in sectioned sagittal otoliths; otoliths in the other two species had poor legibility. Ring formation frequency was determined to be annual. The species becomes a sexually mature male within a year and sex change occurs at about 2.5 years. The maximum age recorded was 9 years. The species is relatively fast-growing and attains similar asymptotic length to other Acanthopagrus species in this region (L∞ = 43.7 cm SL; K = 0.22; t0 = -1.59 for combined sexes).

The trophic ecology of the three study species was investigated using gut contents and stable isotope analysis. All are omnivorous and prey predominantly on small benthic crustaceans. Using the Bayesian Mixing Model for δ13C and δ15N values, resource partitioning among these species was detected. Acanthopagrus schlegelii and adult Pagrus major mainly acquire energy from benthic crustaceans, Evynnis cardinalis from benthic algae and small predators, while juvenile P. major feeds on crustaceans in addition to herbivorous and planktivorous fishes. Seasonal differences in diet content within species were not detected.

Signs of overfishing were detected based on published and unpublished literature, fishermen interviews and market surveys. Although sparids are common in local catches, when compared to past fisheries information, all three study species have clearly declined over recent decades. More than 50% of sparid individuals on sale were below the size of female maturity according to market surveys, indicating biological (growth and recruitment) overfishing.

Their intrinsic life history traits and absence of controls on fishing efforts in Hong Kong and adjacent waters make the three sparids particularly susceptible to overfishing. Thus, there is urgent need to develop conservation and management plans to prevent further declines. Current management measures were considered and suggestions made for management improvement and conservation measures. These included establishment of no-take marine protected areas, safeguarding nursery and spawning grounds, upper and lower size limits, reduction of fishing effort and closed seasons.