Neil Loneragan

Indonesia has the world’s largest shark fishery, but very little locally relevant information is available on catch statistics or life history characteristics of targeted species. This poses major challenges for fisheries management and shark conservation in the region, particularly in the more remote coastal communities of Eastern Indonesia. Shark fishers from three coastal communities were interviewed and trained in data collection in their fishing grounds in the Seram, Aru-Arafura and Timor Seas. Of the over 30 species recorded by the fishers, we assessed ten species based on criteria including proportion of total catch, high intrinsic vulnerability to fishing pressure, and market and conservation value. Using a combination of fishers’ data and published life history characteristics, we estimated the intrinsic rate of population growth to evaluate the relative vulnerability of each species, and compared the observed size structure with established fisheries reference points. The interviews revealed that the fishers generally perceived sharks in their fishing grounds to be declining in numbers and size. Our study found that the fishery is catching several high risk species, such as hammerheads and guitarfish, with catches predominantly comprised of immature individuals and a high level of fishing mortality. Involving fishers in data collection and interpretation provided data and information in regions with low management and research capacity. Additionally, close collaboration with the fishers provided fundamental insights into the circumstances that shape fishers’ decision-making and the ecological and socio-economic requirements that must be addressed for management initiatives to be effective.

Historically the Western School Prawn (Metapenaeus dalli) was an important and iconic catch for commercial and recreational fishers in the Swan-Canning Estuary. However, the abundance of this penaeid declined dramatically since the 1960s, resulting in the closure of the commercial fishery and essentially the cessation of the recreational prawning. In 2012, a restocking and associated research program was initiated to try and increase the abundance of this popular species. Monthly sampling, for two years, of the fish and crustacean faunas of the shallow and deeper waters of this estuary has shown that; 1) the abundance of M. dalli changes markedly throughout the year, particularly in the shallow waters. 2) M. dalli moves into the shallows between November and March to spawn, but that that timing changes slightly among years according to environmental conditions. 3) The distribution of adult and juvenile prawns changes temporally and can be used to inform broodstock collection and release sites. In addition, dietary analysis was performed on 11 abundant fish species in the estuary to determine those species that predate on post-larval M. dalli (direct predators) and those that also target other small crustaceans (potential predators). Key predators included the Gobbleguts Ostorhinchus rueppellii (all sizes) and the Black Bream Acanthopagrus butcheri (<100 mm in length). These dietary results together, with monthly prawn and fish abundance data for 36 sites across the estuary have been used to design an effective release strategy and highlight the value in restocking programs being supported by robust quantitative faunal surveys.

Bioeconomic evaluations of release programs require robust estimates of growth, mortality and reproduction. Length frequency data and mixture analysis were used to estimate biological parameters for the Western School Prawn (Metapenaeus dalli), which is currently being restocked in the Swan-Canning Estuary. Monthly length frequency data, collected from hand and otter trawls on the new moon phase, over 13 lunar cycles, indicated that this species exhibited highly seasonal patterns of growth and reproduction. Growth occurred predominantly in the warmer months (October-April), with little to no growth in the colder months (May-October). The Somers’ seasonal von Bertalanffy growth model estimated that female prawns grew significantly larger (L∞ = 33.72 mm CL, k = 1.06) than their males counterparts (L∞ = 24.20 mm CL, k = 0.97). Gravid females were found only from November to April, when surface and bottom water temperatures ranged from 20 to 27 ºC. Both seasonal and non-seasonal length converted catch curves were used to estimate the instantaneous rate of total mortality Z. The seasonal model demonstrated that female and male mortality were similar (Z = 0.048 and 0.047 week-1, respectively). However, the non-seasonal model showed a greater difference in mortality between females (0.067 week-1) and males and (0.052 week-1). Since fishing mortality is very low, these estimates provide a close approximation for natural mortality M in the population. These results improve our understanding of the biology of M. dalli and will be used in the bioeconomic evaluation of the restocking program.

The abundance of the Western School Prawn (Metapenaeus dalli) declined markedly in the Swan-Canning Estuary after the 1960s, resulting in the initiation of a restocking program in 2012. Further to the decline in abundance, anecdotal evidence from recreational fishers indicated that there had been a spatial shift in the distribution of this prawn. To test the hypothesis that a change in sediment composition may be responsible, the density of M. dalli, recorded monthly between October 2013 and August 2014, was correlated against sediment organic matter content and grain size distribution. Densities of M. dalli were found to differ among sediment types in summer, but not winter. Controlled laboratory experiments were used to investigate whether M. dalli exhibited a preference for sediments from either the upper or lower reaches of their distribution within the estuary. Prawns exhibited a preference for nearshore and offshore sediments that contained a lower percentage contribution of larger grain sizes and/or a higher percentage contribution of finer grain sizes, i.e. sediments from the lower rather than upper reaches. These experiments also revealed that emergence and activity rates of M. dalli are strongly related to photoperiod, with individuals preferring to remain buried during daylight hours and become active during darkness. Visual observations also indicated that the prawns were able to bury more rapidly in finer than coarse sediments, thus reducing their length of exposure to predators. The implications of these findings on the current restocking program for M. dalli in the Swan-Canning Estuary are discussed.

The abundance of the Western School Prawn (Metapenaeus dalli) declined markedly in the Swan-Canning Estuary after the 1960s, resulting in the initiation of a restocking program in 2012. Further to the decline in abundance, anecdotal evidence from recreational fishers indicated that there had been a spatial shift in the distribution of this prawn. To test the hypothesis that a change in sediment composition may be responsible, the density of M. dalli, recorded monthly between October 2013 and August 2014, was correlated against sediment organic matter content and grain size distribution. Densities of M. dalli were found to differ among sediment types in summer, but not winter. Controlled laboratory experiments were used to investigate whether M. dalli exhibited a preference for sediments from either the upper or lower reaches of their distribution within the estuary. Prawns exhibited a preference for nearshore and offshore sediments that contained a lower percentage contribution of larger grain sizes and/or a higher percentage contribution of finer grain sizes, i.e. sediments from the lower rather than upper reaches. These experiments also revealed that emergence and activity rates of M. dalli are strongly related to photoperiod, with individuals preferring to remain buried during daylight hours and become active during darkness. Visual observations also indicated that the prawns were able to bury more rapidly in finer than coarse sediments, thus reducing their length of exposure to predators. The implications of these findings on the current restocking program for M. dalli in the Swan-Canning Estuary are discussed.

This study represents the first attempt to evaluate the ecosystem structure and functioning of the nearshore reefs in the Lidao coastal ecosystem of northern China. This region is one where intensive aquaculture (particularly kelp culture) and fisheries enhancements, through the deployment of artificial reefs and stock enhancement programs are practiced. An Ecopath model was developed for a small area in the region and Ecosim was used to explore one scenario for alternative fishing practices. The mean trophic levels (TL) of the functional groups ranged from 1.0 to 4.1. The mean transfer efficiency was 11.7% through the whole system, and the ecosystem had a relative low maturity, stability and disturbance resistance, remaining at a developing stage. Nearly half of the total system biomass (48.9%) in the system, excluding detritus, (620.20 t km-2 year-1), was comprised of benthic finfish and invertebrates. The total fishery yield from all fisheries (86.82t/km2/year) was dominated by low TL herbivorous and detritivorous species such as the sea cucumber Apostichopus japonicas (TL=2.1, 46.07%), other echinoderms (TL=2.1, 34.6%) (sea urchin Asterias amurensis; Strongylocentrotus nudus) and abalone Haliotis discus hannai (TL=2.0, 18.4%), and as a consequence the mean trophic level of the catch was low (2.1). The Ecosim simulation of closing all fisheries for 20 years (keeping the original state for first 3 years) in the current ecosystem resulted in an increase of about 100% in the relative biomass of the main exploited species, sea cucumber A. japonicas and abalone H. discus hannai.