Instantaneous Total Mortality Research Articles

Age, growth and mortality parameters of Mullus barbatus Linnaeus, 1758 (Perciformes: Mullidae) in Iskenderun Bay, northeastern Mediterranean

The objectives of this study were to determine the age, growth, mortality and exploitation of a commercially important fish of northeastern Mediterranean Sea, Mullus barbatus from the Karatas coast of Iskenderun Bay during September 2002 and April 2003. A total of 212 specimens of red mullet were collected by monthly sampling using traditional bottom trawl net by commercial vessel (Coskun Reis, 22m length and 285HP) from 3 stations in three depth contours in ≤20m, 20-50m and 50-100m, respectively. The age of M. barbatus specimens was estimated by examining sagittal otoliths and it found that the age composition varied from I to V age classes. The mean total length and total weight values was calculated as 10.07±1.83cm and 12.04±8.22g, respectively and the calculated length-weight relationship was W=0.0072*L 3.1618 . The von Bertalanffy growth parameters were L ¥ = 21.98cm, k = 0.194 year-1 and t o = -1.168 year. The instantaneous rate of total mortality ( Z ) was 1.39, the natural mortality ( M ) was 0.45 and the fishing mortality ( F ) was estimated to be 0.93. The exploitation rate ( E ) was calculated as 0.67 using value of M and F . As a result of the analysis, the population of M. barbatus from the Karatas coast of Iskenderun Bay was exploited over the optimum level because of overfishing pressure.

Population dynamics parameters of narrow-barred Spanish mackerel, Scomberomorus commerson (Lacèpéde, 1800), from commercial catch in the northern Persian Gulf

Population dynamics parameters of Scomberomorus commerson in Bushehr area waters of Iran were analyzed between October 2011 and September 2012. Fork length frequencies were collected from the gill net commercial catch. Von Bertalanffy growth function was used to estimate growth parameters K and L∞. Instantaneous total mortality rate (Z), instantaneous natural mortality rate (M) and the instantaneous fishing mortality rate (F) were also calculated. Resource status was evaluated by comparing estimates of the fishing mortality rate with target (Fopt) and limit (Flimit) biological reference points. FiSAT program was used to assess growth and mortality parameters. Based on the growth curve analysis, growth parameters were: K=0.5year−1 and L∞=148cm. Instantaneous total mortality was Z=0.97year−1. The estimate of M=0.56year−1 and F=0.41year−1, resulting in an exploitation rate (F/Z) of E=0.42year−1. Target and limit biological reference points were: Fopt=0.28year−1 and Flimit=0.37year−1. Size at capture at probabilities of 0.25 (L25), 0.5 (L0.5) and 0.75 (L0.75), were 46.3 cm, 55 cm and 60.2 cm respectively. Longevity (Tmax) was calculated as 3/K equal to 6 years. The exploitation rate (E) did not indicate overfishing; however, Fopt and Flimit rates were lower than fishing mortality (F), suggesting that overexploitation occurred. Growth parameters in the present study showed that the gillnet fishery catches of S. commerson that are of ages 1 or 2, which may not have reached maturity. Gillnet mesh size can be a factor why the catch is composed of mostly small, immature fish. Therefore, changes in mesh size and net size should be considered.

Population dynamics of notopterus notopterus (Pallas, 1769) from the Kaptai reservoir of Bangladesh

Population parameters of Notopterus notopterus (Pallas, 1769) from length-frequency data collected from Kaptai Reservoir of Rangamati, Bangladesh from January, 2013 to December, 2013 were calculated by using FiSAT software. The asymptotic total length (L?) and growth coefficient (K) were estimated to be 34.91cm and 0.38 y?1respectively. The instantaneous rate of natural mortality (M), fishing mortality (F) and total mortality (Z) were estimated to be as 0.91 y?1, 0.28 y?1and 1.19 y?1 respectively. The value of exploitation rate (E) was found to be 0.24 which clearly pointed toward moderate or less fishing pressure (E<0.50) of N. notopterus in the Kaptai reservoir of Bangladesh. The recruitment of the species was found throughout the year with two peaks - one from March-April; and another from May- June. Virtual population analysis (VPA) estimated that the maximum numbers of N. notopterus were caught between 10.50 cm to 32.50 cm with maximum F value (0.80 y?1) in the mid length of 18.30 cm. During the year 2013 the total catch of N. notopterus was found to be 664.5 MT, which contributed about 7.78% of the total catch of Kaptai Lake. Relative yield per recruit (Y’/R) and biomass per recruit (B’/R) suggested that the natural mortality should be reduced to 0.91 y?1 by proper management to obtain a maximum sustainable exploitation rate (Emax= 0.828) for the species in the reservoir. DOI: http://dx.doi.org/10.3329/sja.v12i2.21923 SAARC J. Agri., 12(2): 112-122 (2014

Estimating time-based instantaneous total mortality rate based on the age-structured abundance index

The instantaneous total mortality rate (Z) of a fish population is one of the important parameters in fisheries stock assessment. The estimation of Z is crucial to fish population dynamics analysis, abundance and catch forecast, and fisheries management. A catch curve-based method for estimating time-based Z and its change trend from catch per unit effort (CPUE) data of multiple cohorts is developed. Unlike the traditional catch-curve method, the method developed here does not need the assumption of constant Z throughout the time, but the Z values in n continuous years are assumed constant, and then the Z values in different n continuous years are estimated using the age-based CPUE data within these years. The results of the simulation analyses show that the trends of the estimated time-based Z are consistent with the trends of the true Z, and the estimated rates of change from this approach are close to the true change rates (the relative differences between the change rates of the estimated Z and the true Z are smaller than 10%). Variations of both Z and recruitment can affect the estimates of Z value and the trend of Z. The most appropriate value of n can be different given the effects of different factors. Therefore, the appropriate value of n for different fisheries should be determined through a simulation analysis as we demonstrated in this study. Further analyses suggested that selectivity and age estimation are also two factors that can affect the estimated Z values if there is error in either of them, but the estimated change rates of Z are still close to the true change rates. We also applied this approach to the Atlantic cod (Gadus morhua) fishery of eastern Newfoundland and Labrador from 1983 to 1997, and obtained reasonable estimates of time-based Z.

Dispersal and population-level mortality of Micropterus salmoides associated with catch and release tournament angling in a South African reservoir

This study assessed the impact of catch and release tournament angling on a largemouth bass (Micropterus salmoides) population in Wriggleswade Dam, a 1000ha reservoir in South Africa. Effort was estimated at 648 boat days/year and catch as 2236 fish/year based on tournament catch data (2005–2012). Initial mortality rate (dead fish at the tournament weigh in) averaged 0.53%/yr. Dispersal was estimated from the recapture locations of 124 of 786 fish tagged and released during tournaments. The distance of the recapture locality from the release site (dispersal distance) was positively correlated with time at liberty for the first 50 days following release, after which recapture localities were random (average 4km from the release site). At a population level, the instantaneous rate of total mortality (Z=0.36–0.50yr−1) was lower than in many other fisheries indicating a negligible impact of catch and release tournament angling at the effort levels experienced by this fishery.

Population dynamics of the garfish,Belone euxini(Belonidae: Belone) from the south-east Black Sea

To describe the age, growth, mortality and distribution of the garfish,Belone euxiniand differences in these parameters from other populations, monthly samplings were conducted between December 2011 and July 2013 in the south-east Black Sea. Garfish catch decreased to its lowest levels in spring months, especially after May, and it was completely finished by the summer months. The mean total length (TL) of the female was significantly greater than the mean TL of the male (P < 0.05). The sex-ratio of female:male with size was skewed towards males up to 38 cm length-class, and skewed towards females above the 38 cm class (P < 0.05). The growth pattern of garfish was described by goodness of fit of the data and biological interpretation of growth parameters. The exponential (females:L∞ = 88.3 cm TL,K = 0.108 yr−1, males:L∞ = 71.9 cm TL,K = 0.151 yr−1) and von Bertalanffy (females:L∞ = 81.6 cm TL,K = 0.125 yr−1,t0 = −2.25 year, males:L∞ = 81.6 cm TL,K = 0.125 yr−1,t0 = −2.13 year) models were considered the most suitable growth models. Total instantaneous mortality rate,Z, was estimated as 1.04 yr−1for females and 1.24 yr−1for males. Instantaneous natural mortality ratio,M, was estimated higher for males than for females. The exploitation rate of both sexes was also higher than the optimum exploitation (E = 0.5) criterion, which is indicative of overfishing of the garfish population.

Tag shedding by tropical tunas in the Indian Ocean and other factors affecting the shedding rate

A key objective of the Regional Tuna Tagging Project—Indian Ocean was to estimate tag-shedding rates, Type-I (immediate tag shedding) and Type-II (long-term tag shedding). To assess this, a series of double-tagging experiments (26,899 double tags released with 4555 recoveries) were conducted as part of the broader tagging program. After omitting data from tags placed by less experienced taggers, the results of our analyses did not show any evidence that individual differences between taggers (i.e., a tagger effect) impacted estimates of tag-shedding rates. However, it was shown that the probability of retaining the second tag (inserted in the left side of the fish) was larger than retaining the first tag (inserted in the right side, i.e., the side typically tagged in single-tagging experiments). We used a Bayesian model averaging approach to account for model uncertainty in the estimates of the parameters α and L used to calculate the probability of tag retention Q(t)=α e−(L t) for the right tag. The parameter estimates were α=0.993 and L (per year)=0.030 (skipjack); α=0.972 and L (per year)=0.040 (yellowfin); and α=0.990 and L (per year)=0.021 (bigeye). These results agree with estimates obtained by other large-scale tropical tuna tagging projects. We showed that tag loss has a moderate impact on the underestimation of the exploitation rate (bias=2–6% depending on the tuna species). However, non-reporting leads to a bias of around 7% when using the high reporting rate estimate of purse seiners. Finally, tag shedding (specifically Type-II shedding) modified the individual weights of the samples of recaptures. Consequently, the total instantaneous mortality estimates (Z; calculated from mean times-at-large) were reduced by a range of 1–3%.

Growth and mortality of two small fishes, Toxabramis swinhonis Günther, 1873 and Hyporhamphus intermedius (Cantor, 1842), in a Yangtze shallow lake (China) assessed by length frequency analysis

This study examined the growth and mortality of two widespread small fish species, Toxabramis swinhonis (Cyprinidae) and Hyporhamphus intermedius (Hemiramphidae), from a shallow lake (Niushan Lake, China) along the Yangtze River, by analyzing monthly length frequency data (July 2005 to June 2006) with the FAO-ICLARM stock assessment tool (FiSAT). Estimated von Bertalanffy growth equation parameters were: L (asymptotic length)=145mm TL, and K (growth coefficient)=0.66year(-1) for T.swinhonis; L=189mm TL, and K=0.68year(-1) for H.intermedius. The instantaneous rates of natural mortality (M) calculated for T.swinhonis and H.intermedius were 1.35 and 1.28year(-1), respectively. The length-converted catch curve method gave the total annual instantaneous mortality rate (Z) of 2.92year(-1) for T.swinhonis and 1.53year(-1) for H.intermedius. Recruitment patterns of both species were continuous, displaying a single major peak event per year. Despite not being of direct fishing interest, T.swinhonis, with high abundance and productivity as indicated by a high Z-value, can be an important species in fisheries management as an excellent food source for aquatic organisms, especially for larger fishes of economic value. These results represent the first information on population characteristics of T.swinhonis and H.intermedius, and constitute a comparison base for future growth studies in other lake environments.

Impacts of Aging Error on Walleye Management in the Winnebago System

AbstractThe age data used to manage Walleye Sander vitreus fisheries are not always accurate, as otoliths typically provide more accurate age estimates for larger, older Walleyes than dorsal spines. We assessed the impacts that the aging error associated with the use of dorsal spines has on the estimated age distribution, growth and mortality rates, and yield per recruit for Walleyes in the Winnebago system, Wisconsin. Age distributions derived from otolith age estimates more accurately portrayed variable recruitment than those derived from dorsal spine age estimates. The mean estimates of instantaneous total annual mortality developed from sex‐specific catch curves were greater when dorsal spine age estimates were used (0.515 for males, 0.493 for females) than when otolith age estimates were (0.349 for males, 0.396 for females), with most of the differences being observed in natural mortality estimates. The von Bertalanffy growth models were not significantly different, but the yield‐per‐recruit models (which rely heavily on von Bertalanffy model parameters) incorporating spine age data produced average yield estimates 50–70% lower than the models developed from otolith ages. The yield‐per‐recruit models derived from spine‐determined ages also resulted in lower recommendations as to the age and size of fish for maximum yield at all exploitation levels than the models developed from otolith age data. The mean dorsal spine and otolith age estimates were similar for male Walleyes <457 mm and female Walleyes <508 mm, but for larger fish otoliths yielded significantly older ages than spines. Given the economic and social impacts of the Winnebago system Walleye fishery, we recommend exclusive use of otoliths to obtain the most accurate estimates of fish age, growth, mortality, and yield. In populations in which the sacrifice of large quantities of fish is not acceptable, we recommend using spines to estimate the ages of smaller fish (males <457 mm, females <508 mm) and otoliths to estimate the ages of fish larger than these critical lengths.Received December 26, 2012; accepted June 10, 2013

Length-weight relationship, parameters of growth and mortality for the hardhead halfbeak Chriodorus atherinoides Goode & Bean, 1882 (Beloniformes: Hemiramphidae) from Yucatan Peninsula, Mexico

Summary This study reports length–weight relationships and growth parameters for Chriodorus atherinoides from La Carbonera, a karstic tropical coastal lagoon on the northwestern coast of the Yucatan Peninsula, Mexico. A total of 667 specimens were collected between April 2009 and March 2010. The length-weight relationship obtained was W = 0.0034Lt3.22. This study presents the first estimation of the model parameters of the von Bertalanffy growth equation (L∞ = 27.30 cm, k = 0.76 year−1 and t0 = −0.178 years), the instantaneous total mortality (2.51 year−1) and the maximum length record (26.0 cm total length), which is greater than previously recorded.

Population parameters of stargazer (Uranoscopus scaber Linnaeus, 1758) in the southeastern Black Sea region during the 2011-2012 fishing season

Summary The population parameters of stargazer (Uranoscopus scaber Linnaeus, 1758) were studied regarding age composition, sex ratio, growth, survival and mortality rates, and the exploitation rate in the southeastern Black Sea near the coast of Turkey during the 2011–2012 fishing season. According to ageing analysis of the samples, age group 1 is the most abundant (47.76%), followed by age 2 (41.04%), age 3 (9.33%), age 0 (1.12%) and age 4 (0.75%). Mean totals as well as total lengths and weights for males and females were 14.5 ± 0.22 cm and 12.9 ± 0.19 g, 16.6 ± 0.24 cm and 62.1 ± 2.76 g, and 37.2 ± 1.92 cm and 87.1 ± 3.88 g, respectively. The mean condition factor was K = 0.0167, while the sex ratio was 53.98% female, 38.75% male and 7.27% immature. Length-weight, age-length and age-weight equations were W = 0.014 × L3.059, L(t)=44.5*[1−e−0.148*(t +1.242)] and W(t)=1544.4*[1−e−0.148*(t + 1.242)]3.059, respectively. The computed survival rate (S), instantaneous total mortality rate (Z), annual mortality rate (A), natural mortality rate (M) and fishing mortality rate (F) were S = 28.94%, Z = 1.24 year−1, A = 71.06%, M = 0.26 year−1 and F = 0.98 year−1, respectively. The exploitation rate was 0.79, which is above the optimum exploitation levels.

Age, growth and mortality of anchovy Engraulis encrasicolus in the south-eastern region of the Black Sea during the 2010–2011 fishing season

In this research age composition, sex-ratio, growth, survival, mortality and exploitation rates were determined in the south-eastern Black Sea region of Turkey during the 2010–2011 fishing season. Anchovy in the age group 2 was the most abundant (64.06%), followed by age groups 1 (33.12%), 3 (1.80%) and 0 (1.02%). Mean length and weight of the total sample, males and females, were 11.63 ± 0.02 cm, 9.98 ± 0.04 g; 11.39 ± 0.03 cm, 9.40 ± 0.07 g; and 11.98 ± 0.02 cm, 10.73 ± 0.05 g, respectively. The mean condition factor was CF = 0.63 and the sex composition was 61.27% female, 34.40% male and 4.33% unidentified. Equations describing the relationship between length and weight, age and length, and age and weight were derived as W = 0.011 × L2.742, L(t) = 16.368*(1–e−0.425*(t +1.35)) and W(t) = 23.516*(1 –e–0.425*(t+1.35))2,747, respectively. The survival (S), instantaneous total mortality (Z), annual mortality (A), natural mortality (M) and fishing mortality (F) rates were found to be S = 5.8%, Z = 2.84 yr–1,, A = 94.2%, M = 0.66 yr−1, and F = 2.18 yr–1, respectively. The exploitation rate was calculated as 0.77, which is higher than the optimum exploitation level.

Evaluation of Age–Length Key Sample Sizes Required to Estimate Fish Total Mortality and Growth

AbstractFisheries assessments often rely on age composition data to infer information about growth, mortality, and the outcome of various management policies. To reduce the costs of estimating the age of all fish collected (i.e., via direct aging), an age–length key (ALK) is often developed by directly aging only a subsample of fish and estimating the ages of the remaining fish based on observed lengths. Although this approach is common, there is little guidance in the literature for determining the appropriate total sample size and numbers of fish needed to directly age for each length bin. We developed a stochastic simulation to evaluate the sample‐size requirement for using ALKs to estimate von Bertalanffy growth parameters and the instantaneous rate of total mortality (Z). The simulations accounted for variation in life history characteristics of stocks and exploitation rates of fisheries. Our simulations suggested that for overfished populations, aging more fish per bin provides negligible benefits whereas collecting larger total sample sizes could improve bias and precision ofZestimates. These results also indicated that bias and precision was relatively uninfluenced by fish life history type, which may allow for standardization of field collection methods across a wide range of fish species. In most cases, total sample sizes of 500–1,000 and 10 fish aged per bin provided near optimal performance in accuracy and precision, suggesting that this could be a guideline for many studies.

Age-based demography and relative fisheries productivity of Spanish mackerel, Scomberomorus commerson (Lacepede) in Western Australia

Spanish mackerel, Scomberomorus commerson, is a species of economic and artisanal importance throughout the Indo-West Pacific region. In this study a total of 2973 S. commerson were examined in a comprehensive study of otolith structure and demographic characteristics across multiple management regions along the Western Australian (WA) coast. Marginal increment analysis of sagittal otoliths showed a single annual peak in opaque zone formation indicating that one opaque and translucent band (annuli) is formed each year. Juvenile S. commerson of undifferentiated sex (n=67; 58–709mm FL) ranged in age from 23 days to 350 days. Males (n=1239; 301–1381mm FL) ranged in age from 80 days to 22yrs, and females (n=1273; 396–1720mm FL) ranged in age from 122 days to 18yrs. Significant differential growth was evident between sexes, with females attaining a larger size-at-age than males. Initial growth of S. commerson is rapid with both sexes reaching maturity within 1.5yrs of age. Growth parameters also differed among regions. The instantaneous rate of natural mortality (M) was estimated to be in the range of 0.16–0.20yr−1. The instantaneous rate of total mortality (Z) estimated from catch at age data for fully recruited age classes, was different between sexes and among regions. Significant differences in demography, age structure and otolith readability among management regions support earlier studies that have documented meta-population structure in S. commerson and underpin the regional management regime for this species in Western Australia. Populations of S. commerson have a high production potential given their rapid initial rate of growth and low age at maturity. Despite this high production potential, productivity of this species in WA is relatively low compared to related fisheries throughout the Indo-Pacific region.

Exploring Mechanisms Underlying Sex‐Specific Differences in Mortality of Lake Michigan Bloaters

AbstractSex‐specific differences in mortality rates have been observed among freshwater and marine fish taxa, and underlying mechanisms can include sex‐specific differences in (1) age at maturity, (2) growth rate, or (3) activity or behavior during the spawning period. We used a long‐term (1973–2009) Lake Michigan data set to evaluate whether there were sex‐specific differences in catch per unit effort, mortality, age at maturity, and length at age in bloaters Coregonus hoyi. Because bloater population biomass varied 200‐fold during the years analyzed, we divided the data into three periods: (1) 1973–1982 (low biomass), (2) 1983–1997 (high biomass), and (3) 1998–2009 (low biomass). Mortality was higher for males than for females in periods 2 and 3; the average instantaneous total mortality rate (Z) over these two periods was 0.71 for males and 0.57 for females. Length at age was slightly greater (2–6%) for females than for males in different age‐classes (3–6 years) during each period. Age at maturity was earlier for males than for females in periods 1 and 2, but the mean difference was only 0.2–0.4 years. To test the hypothesis that somatic lipids declined more in males than in females during spawning (perhaps due to increased activity or reduced feeding), we estimated sex‐specific percent somatic lipids for fish sampled in 2005–2006 and 2007–2008. During 2005–2006, somatic lipids declined from prespawning to postspawning for males but were unchanged for females. During 2007–2008, however, somatic lipids were unchanged for males, whereas they increased for females. We found that sex‐specific differences in Z occurred in the Lake Michigan bloater population, but our hypotheses that sex‐specific differences in maturity and growth could explain this pattern were generally unsupported. Our hypothesis that somatic lipids in males declined during spawning at a faster rate than in females will require additional research to clarify its importance.

Validation of Annulus Formation in Otoliths of Notchlip Redhorse (Moxostoma collapsum) and Brassy Jumprock (Moxostomasp.) in Broad River, South Carolina, with Observations on their Growth and Mortality

Moxostoma collapsum (Notchlip Redhorse) and the undescribed Moxostoma sp. (Brassy Jumprock) form a large component of the fish assemblage in the Broad River, SC, but little is known about their population characteristics, nor has an aging method been validated for either species. We validate that one annulus is formed each year in the lapillus otolith of the two species and identify spring as the period of annulus formation, using chemical marking and marginal increment analyses, respectively. Notchlip Redhorse and Brassy Jumprock are long-lived, with observed maximum estimated ages of 17 and 21, respectively. Both species grow quickly during their first few years; little growth occurs after age 7. Instantaneous total mortality of each species was low (Z ≤ 0.35), suggesting there is little exploitation of either species in the Broad River, SC.

울산 태화강 바지락의 자원평가 및 관리방안에 관한 연구

The manila clam (Ruditapes philippinarum) is mainly distributed in the coastal area which consist of mud, sand and gravel, but they rarely live on the upper and down reaches of river. For a long time the manila clam has been inhabited in Taehwa river which has been exploited as a traditional earning resources and has become as a major object by neighborhood fishermen. This study was undertaken to evaluate stock assessment and to build management implications with the ecological parameters in Taehwa river from June 2009 to June 2010. The maximum age of manila clam was determined to 6 years old from observing ring radius of shell, the length and weight relationship was TW = 0.0002SL 3.063 (R 2 = 0.925). K and L∞ were respectively estimated 46.64 mm and 0.341/year by von Bertalanffy growth. The instantaneous total mortality was estimated to be 1.171/year and the age at first capture was 1.37 years by the Pauly's method using shell length composition. The current total biomass of manila clam was calculated 1,483 mt over study area 1.46 km 2 by swept area method. ABC (Acceptable Biological Catch) estimates of manila clam showed 512 mt with using F0.1. It's desirable to determine the optimum harvesting time as after main spawning season, as well as it's required to manage fisheries resources considering capture age and biomass through adjusting a first age at capture.

Indications of Continued Overexploitation of Speckled Hind Along the Atlantic Coast of the Southeastern United States

AbstractFrom 1977 to 2007, speckled hind Epinephelus drummondhayi were sampled along the Atlantic coast of the southeastern United States from North Carolina to central Florida. In total, 1,365 speckled hind were obtained from two sources (fishery‐independent survey and commercial catches). The ages of specimens were estimated from increment counts on transverse sections of sagittal otoliths. Sex and reproductive state of specimens were determined by histological examination of gonads. Age of collected speckled hind ranged from 1 to 35 years, and total length (TL) ranged from 164 to 973 mm. The TL and age ranges were 169–730 mm and 1–13 years, respectively, for females; 366–730 mm and 2–13 years for mature females; 451–718 mm and 3–7 years for transitional individuals; and 500–930 mm and 4–35 years for males. There were several indications that the speckled hind population off the U.S. Atlantic coast continues to be overexploited despite a trip limit of 1 fish/vessel, which was established in 1994. The instantaneous total mortality rate (Z) and fishing mortality rate (F) increased from 1979–1981 (Z = 0.61; F = 0.48) to 2004–2007 (Z = 1.27; F = 1.14). Mean age and mean TL also decreased significantly from 7.2 years and 574 mm in 1979–1981 to 4.5 years and 462 mm in 2004–2007. No individuals above the age of 10 were collected during 2004–2007. Mean length at ages 4–6 decreased significantly from 1979–1981 to 2004–2007. During 2004–2007, 73% of specimens were immature females, and none of the sampled females was in spawning condition.

Age, growth and mortality of Clarias gariepinus (Siluriformes: Clariidae) in the Mid-Cross River-Floodplain ecosystem, Nigeria

Clarias gariepinus is a threatened highly prized species used for some elite ceremonies by the local communities. Artisanal fishers take advantage of this species annual breeding migration from the lower Cross River to the floodplain lakes in Mid-Cross River during the rainy season, and some migrant stocks are not able to spawn. Since there is a lack of information on this species population dynamics in the Mid-Cross area, this study aimed to evaluate the age, growth and mortality to support the development of effective management plans. For this, monthly overnight gill net catches (from 6 to 72mm mesh sizes) were developed between March 2005 and February 2007. Growth parameters were determined using the FiSAT II length-frequency distribution. A total of 1 421 fish were collected during the survey. The asymptotic growth (L(infinity)) was 80.24cm, growth rate (K) was 0.49/year while the longevity was 6.12 years. The annual instantaneous rate of total mortality (Z) was 2.54/year and the natural mortality (M) was 0.88. Fishing mortality (1.66/year) was higher than the biological reference points (F(opt) = 0.83 and F(limit) = 1.11) and the exploitation rate (0.66) was higher than the predicted value (E(max) = 0.64) indicating that C. gariepinus was over exploited in the Mid-Cross River-Floodplain ecosystem. Some recommended immediate management actions are to strengthen the ban of ichthyocide fishing, closure of the floodplain lakes for most of the year, restricted access to the migratory path of the fish during the flood period and vocational training to the fishers. In order to recover and maintain a sustainable harvest, I suggest that a multi-sector stakeholder group should be formed with governmental agents, community leaders, fishers, fisheries scientists and non-governmental organizations. These short and long term measures, if carefully applied, will facilitate recovery of the fishery.

Age, growth, and mortality of Loligo vulgaris wild paralarvae: implications for understanding of the life cycle and longevity

Abstract González, Á. F., Otero, J., Pierce, G. J., and Guerra, Á. 2010. Age, growth, and mortality of Loligo vulgaris wild paralarvae: implications for understanding of the life cycle and longevity. – ICES Journal of Marine Science, 67: 1119–1127. Age, growth, and mortality were estimated for the first time in wild paralarvae of the common squid, Loligo vulgaris, by examining growth increments in the statoliths of 273 animals collected off the Ría de Vigo (NW Spain, NE Atlantic). Hatching was all year round for the period 2003–2005, with a peak during late spring and a secondary peak during early autumn. Paralarvae varied from 1260 to 7580 µm, and their abundance decreased abruptly as they grew. Statolith increments were clearly visible without grinding in almost all material, allowing reliable estimation of age. Paralarvae are planktonic for at least 3 months. Growth in dorsal mantle length (DML) during that period fitted an exponential equation. The instantaneous relative growth rates were 2.11, 2.15, and 1.82% DML d−1 for 2003, 2004, and 2005, respectively, and there were no significant differences in size-at-age between the 3 years. Taking into account the growth rates estimated for the whole cycle of L. vulgaris, we suggest that the lifespan may previously have been underestimated by 3 months, because the proximity of the rings deposited during paralarval and early juvenile stages would prevent accuracy in enumerating the number of growth increments in later stages. The estimated instantaneous rate of total mortality during the first 90 d of a paralarva life was 9.6, 5.3, and 4.8% d−1 for 2003, 2004, and 2005, respectively. Eye diameter was a reliable and rapid way of estimating DML and age.