The biology of elasmobranchs makes them very vulnerable to fishing pressure and there is increasing international concern over their exploitation. In northern Australia the stocks of some species may be shared with those in southern Indonesia. Indonesia has the highest landings of elasmobranchs worldwide ([100,000 t p.a.) and millions of Indonesian artisanal fishers rely heavily on elasmobranchs taken in target fisheries. They are also taken by industrial trawlers and as bycatch in pelagic tuna fisheries. This paper, resulting from a collaborative project between Australia and Indonesia, summarises the elasmobranch fisheries; the characteristics of the fisheries are outlined, the status of the stocks are assessed, and management options described and discussed. The project focussed on representative markets and fish landing sites in southern Indonesia from 2001 to 2005. Data were from market surveys, the records of the Indonesian Directorate General of Capture Fisheries, and from research cruises. Data from the ongoing tuna monitoring programme showed that shark bycatch from the tuna fleets forms about 11% of shark landings in Indonesia. Yield per recruit and related analyses were used to integrate biological information to indicate the productivity of each species to allow for management policy options and constraints. Research cruise data show that catch rates of elasmobranchs in the Java Sea declined by at least one order of magnitude between 1976 and 1997. The results indicate strongly that many of the shark and ray species in Indonesia are overfished and that the most effective management strategy may need to involve capacity control, such as licencing, gear restrictions and catch limits, together with controls on the fin trade.
The demographics of the southern bluefin tuna (SBT) Thunnus maccoyii spawning stock were examined through a large-scale monitoring program of the Indonesian longline catch on the spawning ground between 1995 and 2012. The size and age structure of the spawning population has undergone significant changes since monitoring began. There has been a reduction in the relative abundance of larger/older SBT in the catch since the early 2000s, and a corresponding decrease in mean length and age, but there was no evidence of a significant truncation of the age distribution. Pulses of young SBT appear in the catches in the early- and mid-2000s and may be the first evidence of increased recruitment into the spawning stock since 1995. Fish in these two recruitment pulses were spawned around 1991 and 1997. Size-related variations in sex ratio were also observed with female bias for fish less than 170 cm FL and male bias for fish greater than 170 cm FL. This trend of increasing proportion of males with size above 170 cm FL is likely to be related to sexual dimorphism in growth rates as male length-at-age is greater than that for females after age 10 years. Mean length-at-age of fish aged 8–10 years was greater for both males and females on the spawning ground than off the spawning ground, suggesting that size may be the dominant factor determining timing of maturation in SBT. In addition to these direct results, the data and samples from this program have been central to the assessment and management of this internationally harvested stock.
Red snappers were examined for reproductive biology and age-0 habitat preferences. Spawning in red snappers occurred throughout the year in northern Australia and eastern Indonesia; at least 10-30% of females and 40-
Knowledge of spawning behaviour and fecundity of fish is important for estimating the reproductive potential of a stock and for constructing appropriate statistical models for assessing sustainable catch levels. Estimates of length-based reproductive parameters are particularly important for determining potential annual fecundity as a function of fish size, but they are often difficult to estimate reliably. Here we provide new information on the reproductive dynamics of southern bluefin tuna (SBT) Thunnus maccoyii through the analysis of fish size and ovary histology collected on the spawning ground in 1993–1995 and 1999–2002. These are used to refine previous parameter estimates of spawning dynamics and investigate size related trends in these parameters. Our results suggest that the small SBT tend to arrive on the spawning ground slightly later and depart earlier in the spawning season relative to large fish. All females were mature and the majority were classed as spawning capable (actively spawning or non-spawning) with a very small proportion classed as regressing. The fraction of females spawning per day decreased with fish size, but once females start a spawning episode, they spawned daily irrespective of size. Mean batch fecundity was estimated directly at 6.5 million oocytes. Analysis of ovary histology and ovary weight data indicated that relative batch fecundity, and the duration of spawning and non-spawning episodes, increased with fish size. These reproductive parameter estimates could be used with estimates of residency time on the spawning ground as a function of fish size (if known) and demographic data for the spawning population to provide a time series of relative annual fecundity for SBT.
Abalon merupakan salah satu jenis moluska laut yang memiliki nilai ekonomis penting dan spesies unggulan untuk budidaya di Indonesia. Keberhasilan budidaya abalon ditentukan oleh ketersediaan benih yang memadai. Selain itu, ketersediaan pakan yang baik juga berpengaruh nyata terhadap pertumbuhan abalon. Efektivitas pakan buatan untuk pembesaran abalon dipengaruhi oleh bahan baku yang digunakan. Untuk itu, penelitian ini bertujuan mendapatkan informasi dasar tentang bahan baku untuk pengembangan pakan buatan dan respons pertumbuhan abalon H. squamata terhadap pakan buatan. Bahan baku pakan dikoleksi, dianalisis komposisi nutriennya yang meliputi komposisi proksimat, asam amino, dan asam lemaknya. Percobaan aplikasi pakan buatan berbahan baku rumput laut dilakukan di Laboratorium Pakan, Balai Besar Penelitian dan Pengembangan Budidaya Laut, Gondol. Empat pakan percobaan dengan kombinasi tepung rumput laut berbeda dibuat dalam bentuk pelet kering. Sebagai pakan kontrol digunakan Gracilaria sp. segar dari tambak. Benih abalon ukuran bobot rata-rata 3,07±1,11 g dengan panjang cangkang 2,79±0,27 cm dipelihara dalam bak beton ukuran 2 m x 0,5 m x 0,5 m dengan kepadatan 400 ekor/bak. Bak dilengkapi dengan sistem air mengalir dan aerasi. Abalon diberi pakan percobaan dua kali sehari, pagi, dan sore hari. Percobaan dirancang dengan rancangan acak lengkap dengan lima perlakuan dan tiga ulangan untuk setiap perlakuan. Pada penelitian ini diperoleh empat jenis rumput laut yang potensial dapat dimanfaatkan sebagai bahan baku pakan abalon H. squamata. Hasil uji pakan menunjukkan bahwa pakan dari campuran tepung rumput laut Gracilaria sp. tambak dengan Ulva sp. (2:1) atau campuran Gracilaria sp. tambak dengan Sargassum sp. (2,3:1) memberikan respons pertumbuhan abalon H. squamata yang baik.
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