An account of fisheries development in the Republic of Djibouti with notes on the growth and mortality of three species of groupers

A brief description of fisheries development in Djibouti is given, with emphasis on the major constraints that have to date limited the increase of fishing effort. Estimates of L sub( infinity ) obtained through Wetherall plots are presented for three important demersal species caught off northern Somalia and landed in Djibouti: the groupers Cephalopholis sonnerati, Epinephelus chlorostigma and E. areolatus (Fam. Serranidae). These are combined with estimates of the growth performance index O' to calculate K values, subsequently used for the construction of length-converted catch curves. The estimate of mortality thus obtained suggests that these stocks are lightly fished.

A reassessment of equations for predicting natural mortality in Mediterranean teleosts

This brief article presents new empirical models for prediction of natural mortality (M) from growth parameters (L and K, W and K) in Mediterranean teleosts, based on 56 data sets presented in an earlier paper in the January 1993 issue of Naga, the ICLARM Quarterly in which models were presented that included temperature as a predictor variable, although its effect was nonsignificant and its partial slope had the "wrong" sign.

Empirical equations for the estimation of natural mortality in Mediterranean teleosts

Empirical relationships were established linking estimates of the instantaneous rate of natural mortality (M), the von Bertalanffy growth parameters, L sub( infinity ) (or W sub( infinity )) and K, and annual mean water temperature in 56 stocks of Mediterranean teleosts fish. It is suggested that these relationships generate for these fish more reliable estimates of M than the widely-used model of Pauly (1980, J. Cons. CIEM 33(3):175-192), which was based on 175 fish stocks, but included only five stocks from the Mediterranean.

Evaluating mortality using length-frequency data when growth parameters are poorly known

A method is presented through which the total mortality undergone by several fish stocks of the same species can be compared when growth parameters are poorly known or unknown. Whereas the estimate of Z obtained via the length-converted catch curve is highly sensitive to the input parameters K and L sub( infinity ), the ratio of Z estimates obtained for different stocks with the same combination of parameters is almost independent of these inputs, at least when the fit of the linear regression is good. The method is tested on simulated data and an application is presented using real data from the Lesser Antilles. It provides the possibility of qualitatively comparing several stocks in situations of scarce biological knowledge.

On the growth, mortality and recruitment of the spiny lobster (Panulirus homarus) in Sri Lankan waters

Monthly length-frequency data of spiny lobster Panulirus homarus collected from the south coast of Sri Lanka during 1988-1990 were analyzed to estimate von Bertallanfy growth parameters. The asymptotic lengths estimated using Wetherall plots were 322 mm and 315 mm total length for the males and females, respectively. Using o' values of 3.53 for males and 3.61 for females, the growth constant (K) was estimated as 0.21 year super(1) and 0.27 year super(1) for the males and females, respectively. The estimates of natural and total mortality (M and Z) are 0.98 year super(1), 1.96 year super(1) for males and 0.92 year super(1), 1.54 year super(1) for females respectively. Recruitment appears to occur in two pulses per year.

Growth, mortality, maturity and length-weight parameters of fishes in Lake Kariba, Africa

This preliminary compilation presents vital parameters for 22 species of freshwater fish from Lake Kariba. The majority of the growth parameters are derived from tables in Balon and Coche's "Lake Kariba: a man-made tropical ecosystem in central Africa". The rest of the parameters are compiled from more recent sources and unpublished data.

VPA estimates of fishing mortality and exploited biomass from Sardinella aurita catch-at-length data in eastern Venezuela

In this study, length-frequency data on Spanish sardine (Sardinella aurita) from northeastern Venezuela were analyzed for the period 1967-1989. Average growth parameters for the von Bertalanffy equation were established as L sub( infinity )= 26.6 cm (TL) and K = 1.26 year super(-1). The number of recruits to the fishing area, estimated from length-structured Virtual Population Analysis, varied from <10 super(8) in the late 1960s to >10 super(9) at the end of the 1980s. Exploited biomass estimates for the same period varied from less than 20,000 t in the first year to more than 100,000 in 1989. Both recruitment and exploited biomass showed different seasonal patterns between 1976-1983 and 1984-1988. Despite some uncertainty regarding these estimates, it is considered that major population tendencies are adequately represented by this analysis

Growth, mortality and recruitment of Nile perch Lates niloticus (L. Centropomidae) in the Nyanza Gulf of Lake Victoria: an evaluation update

A reassessment of the estimates of growth, mortality and recruitment patterns of Nile Perch, Lates niloticus was made based on data from commercial landings collected during the Catch Assessment Survey Programme. Two sets of length frequency data, one each from beach seining and hook and line fisheries, were analyzed. Values of L8 = 169 and 230 (cm TL) and K= 0.18 yr-1 and 0.195 yr-1 were obtained. The total mortality estimates from the catch curve analysis were Z = 0.72 yr-1 and 0.94 yr-1, respectively, with a natural mortality M of about 0.35 for a mean environmental temperature of 27oC. The highest peak for recruitment was in November, December and January with a minor one in June, indicating recruitment of two cohorts per year. These results are discussed and compared to previously available information on L. niloticus in Lake Victoria.

Growth, mortality and yield-per-recruit of the poor cod, Trisopterus minutus capelanus, from the Strait of Sicily

Length-based methods (LBMs) were used to study the growth of Trisopterus minutus capelanus in the Strait of Sicily (Messina Strait). A total of 16,304 'merluzzetto' or poor cod collected by experimental trawling off the southern coast of Sicily during spring, summer, autumn 1986 and winter 1987 were measured in order to estimate the length structure of the population. Length-frequency distribution were analyzed and normal components were discriminated. Von Bertalanffy growth parameters were derived from the mean length of the normal components. The growth parameters obtained by weighted non-linear regression were: K=0.462 (yr super(1)), L sub( infinity )=222.3 (TL,mm) and t sub(o)=-0.679 yr. The resulting growth performance index ( Phi ') was 4.36, a value slightly lower than those derived for Western Mediterranean (mean Phi '=4.45) and Adriatic ( Phi '=4.58) populations and slightly higher than that derived for Hellenic waters ( Phi '=4.27). On the basis of the von Bertalanffy parameters estimated, an array of age-specific instantaneous natural mortality rate (M sub(t)=0.5-1.1) and an average value of total natural mortality rate (Z=2.1 yr super(1)) were estimated and used in the Thompson and Bell yield per recruit (Y/R) analysis in order to evaluate the status of the fishery and forecast the effects of changes in the fishing pattern. Results indicate that this resource is overexploited and that Y/R could be increased by postponing the age at first capture from 0.5 to 1.0 yr. Even a slight reduction in fishing mortality could improve the performance of the fishery. At the present level of exploitation, and assuming a constant recruitment, the spawning stock biomass per recruit (SPR) is well below the conservative threshold of 30% of the pristine or unexploited SPR.

Growth, mortality and recruitment of Omani abalone (Haliotis mariae)

Historical length-frequency data of Oman abalone (Haliotis mariae) from two areas (Sadh and Hadbin) of the Dhofar coast of the Sultanate of Oman were used to estimate growth parameters by nonlinear least square fitting. The results were verified using the ELEFAN I program and then combined to calculate total mortality (Z) and recruitment patterns. The growth parameters values with combined sexes were L sub( infinity ) = 137 mm shell length (SL), K = 0.75 year super(1) and 1.57 year super(1) on Sadh male and female, respectively. The female Z value in Hadbin was 1.55 year super(1) in 1989/90. The 1991 Z value for combined sexes were 2.37 year super(1) in Sadh and 1.66 year super(1) in Hadbin, showing much higher fishing pressure in recent years. There were two recruitment pulses, a major one in January and a minor one in May.

Relative importance of various predators in Clarias gariepinus fry mortality in Cameroon

To estimate the relative importance of the most common predators of Clarias gariepinus fry, increasing levels of protection were afforded to exclude amphibians, aquatic arthropods and birds. At a stocking density of 10 larvae/sq.m. in nursing ponds, fencing off amphibians resulted in a 28 per cent decrease in mortality. Holding fry in hapas to protect them from both amphibians and aquatic arthropods decreased mortality by an insignificant 5.7 per cent. Installation of bird-netting over the hapas reduced mortality by 21.7 per cent. The remaining 4.9 per cent of total mortality, which could not be explained, was attributed to opportunistic cannibalism, disease and/or handling stress. Increasing stocking density to 40/sq. m. and, thus, reducing the food available per fry increased mortality by 28.3 per cent.

Growth and mortality of dagaa (Rastrineobola argentea, Fam. Cyprinidae) in Lake Victoria

Growth and mortality parameters of the small Lake Victoria cyprinid Rastrineobola argentea were determined from length-frequency analysis, using the ELEFAN I and II programs. The results of two sampling programs, both performed during 1988, one in Uganda (mosquito seine) and the other in Tanzania (pelagic trawl), were highly corresponding, In comparison with previously published data on the growth of dagaa and some similar species, low values for L sub( infinity ) (65 mm standard length) and K (1 year super(-1)) were found. Total mortality (Z) amounted to 3.9-4.4 year super(-1). A single annual breeding peak was observed both in Uganda (October/November) and in Tanzania (February/March).

Age, growth, mortality, and radiometric age validation of gray snapper (Lutjanus griseus) from Louisiana

The gray snapper (Lutjanus griseus) is a temperate and tropical reef fish that is found along the Gulf of Mexico and Atlantic coasts of the southeastern United States. The recreational fishery for gray snapper has developed rapidly in south Louisiana with the advent of harvest and seasonal restrictions on the established red snapper (L. campechanus) fishery. We examined the age and growth of gray snapper in Louisiana with the use of cross-sectioned sagittae. A total of 833 specimens, (441 males, 387 females, and 5 of unknown sex) were opportunistically sampled from the recreational fishery from August 1998 to August 2002. Males ranged in size from 222 to 732 mm total length (TL) and from 280 g to 5700 g total weight (TW) and females ranged from 254 to 756 mm TL and from 340 g to 5800 g TW. Both edge analysis and bomb radiocarbon analyses were used to validate otolith-based age estimates. Ages were estimated for 718 individuals; both males and females ranged from 1 to 28 years. The von Bertalanffy growth models derived from TL at age were Lt = 655.4{1–e[–0.23(t)]} for males, Lt = 657.3{1–e[– 0.21(t)]} for females, and L t = 656.4{1–e[– 0.22 (t)]} for all specimens of known sex. Catch curves were used to produce a total mortality (Z) estimate of 0.17. Estimates of M calculated with various methods ranged from 0.15 to 0.50; however we felt that M= 0.15 was the most appropriate estimate based on our estimate of Z. Full recruitment to the gray snapper recreational fishery began at age 4, was completed by age 8, and there was no discernible peak in the catch curve dome.

Maximum likelihood estimation of mortality and growth with individual variability from multiple length-frequency data

We consider estimation of mortality rates and growth parameters from length-frequency data of a fish stock and derive the underlying length distribution of the population and the catch when there is individual variability in the von Bertalanffy growth parameter L∞. The model is flexible enough to accommodate 1) any recruitment pattern as a function of both time and length, 2) length-specific selectivity, and 3) varying fishing effort over time. The maximum likelihood method gives consistent estimates, provided the underlying distribution for individual variation in growth is correctly specified. Simulation results indicate that our method is reasonably robust to violations in the assumptions. The method is applied to tiger prawn data (Penaeus semisulcatus) to obtain estimates of natural and fishing mortality.

Comparison of two approaches for estimating natural mortality based on longevity

Vetter (1988) noted that her review of the estimation of the instantaneous natural mortality rate (M) was initiated by a discussion among colleagues that identified M as the single most impor ta nt but least well-estimated parameter in fishery models. A lthough much has been accomplished in the inter vening years, M remains one of the most difficult parameters to estimate in fishery stock assessments. A number of novel approaches using tagging and telemetry data provide promise for making reliable direct estimates of M for a given stock (Hearn et al., 1998 ; Frusher and Hoenig, 2001; Hightower et al., 2001; Latour et al., 2003; Pollock et al., 2004). However, such methods are often impracticable and fishery scientists must approximate M by using estimates made for other stocks of the same or similar species or by predicting M from features of the species’ life history (Beverton and Holt, 1959; Beverton, 1963; Alverson and Carney, 1975; Pauly, 1980; Hoenig, 1983; Peterson and Wroblewski, 1984; Roff, 1984; Gunderson and Dygert, 1988; Chen and Watanabe, 1989; Charnov, 1993; Jensen, 1996; Lorenzen, 1996).