Article

Demographics and length and weight relationships of commercially important sharks along the north‐western coast of India

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Abstract

• Biological data including size, sex ratios, male maturity, and length and weight relationships for four commercially important shark species, including the milk shark (Rhizoprionodon acutus Rüppell, 1837), the grey sharpnose shark (Rhizoprionodon oligolinx Springer, 1964), the spadenose shark (Scoliodon laticaudus Muller & Henle, 1838), and the bigeye smoothhound shark (Iago omanensis Norman, 1939), landed in Porbandar, Gujarat, India, are provided. • All four species were landed by trawlers and gill‐netters across three defined seasons, with seasonal differences. Higher proportions of mature R. acutus and S. laticaudus were observed in the pre‐monsoon season, with neonates caught throughout the year, whereas higher proportions of mature R. oligolinx and I. omanensis were recorded during the monsoon season, with neonates caught in post‐monsoon and pre‐monsoon, respectively, showing important species‐level differences. • These small‐bodied shark species (less than 1 m in total length) showed positive allometry in their length and weight relationships. Unlike the other three species, I. omanensis showed high disparity in total lengths (LT) between the sexes, with females being larger than males, and with males maturing faster, with the smallest mature male of 33.58 cm LT. Females outnumbered males except in R. acutus, and pregnant females of all species were recorded at least once. Of the 971 males recorded across species, 55.1% were mature and 44.9% were immature. • Results from this study indicate that there is substantial overlap between the distributions of these species and fishing activities, and show that most, if not all, life stages are susceptible to mortality as a result of fishing. • This study provides managers with a better understanding of the life‐history traits of these commercially important species to support future quantitative population assessments, and provides a baseline of trends in fishing‐related mortality.

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The milk shark, Rhizoprionodon acutus (Rüppel, 1837), is the shark species most abundantly landed at fishing sites along the Senegalese coast in the eastern tropical Atlantic. Adult males and females were mainly captured from March to September. The smallest adult male was 840 mm total length (TL) and weighed 2650 g; all males above 950 mm TL were adult. The smallest adult female was 890 mm TL and 4800 g; all females above 1000 mm were adult. The largest male and female were 1215 mm and 1260 mm TL and 6700 g and 6830 g, respectively. There was a significant difference in the total mass vs. TL relationship between males and females. Parturition and mating occurred in May and June. Gestation lasted approximately one year. Females had an annual reproductive cycle although some reproduced in alternate years. The diameter and mass of the largest yolked oocytes ranged 20-23 mm (mean 21.2±0.9) and 4.1-5.6 g (mean 4.8±0.5). Both uteri were compartmentalized into chambers with a single embryo in each chamber. Size and mass at birth, based on term embryos and neonates, ranged 325-500 mm TL (mean 391.4±24.4) and 127-350 g (mean 220.7±37.9). A chemical balance of development based on mean dry mass of the largest yolked oocytes and term embryos was 23. Ovarian fecundity was slightly higher than uterine fecundity. There was a slight positive relationship between uterine fecundity and female TL, but not between ovarian fecundity and female TL. Litter sizes ranged from one to eight (mean 3.5±1.3) with males and females equally distributed. In free-swimming specimens, females significantly outnumbered males, especially among sub-adult and adult specimens.
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Female Rhizoprionodon acutus were found to mature between 62 and 74 cm total length (LT) whereas males matured between 63 and 71 cm LT. The LT at 50% maturity was 64·3 cm for females and 64·7 cm for males. Litter size varied from one to six embryos, and was positively correlated with maternal LT. Female embryos outnumbered males by a ratio of 2·3:1. The size at birth was c. 37 cm LT. Full-term embryos and post-partum females were observed during all seasons although their occurrence was highest in spring. Spermatozoa were rarely recorded in the oviducal gland, indicating that this species does not store sperm. It was not possible to generate maturity curves for Iago omanensis but it was evident that females matured by the time they reached 35 cm and males were mature by 31 cm LT. This species displayed a clearly defined reproductive cycle with parturition occurring primarily in spring, after a gestation period of c. 1 year. Maximal embryo size was 19 cm LT while maximal litter size was 24 embryos. The oviducal gland appeared to act as a seminal receptacle and it appeared that females may utilize these stores by not mating every year.
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Shark finning - chopping-off the fins and discarding the rest - is increasing worldwide to satisfy the demand of shark-fin soup. This massive requirement for shark fins and other shark-related products has created an industry motivated by high returns. Reaching figures of up to US$ 116/kg, shark fins have become one of the world’s most precious commodities. India has rich resources of elasmobranchs. Of this, annual shark production has been around 45,500 tonnes, obtained as a by-catch from a variety of gears. Shark-fin export in India reached its peak in 1995 with 303 tonnes, while a second peak was in 2001. Indian shark fins have been processed and marketed in many forms. Some of the shark-fin products have large market demand. Intricate techniques used for grading, processing and packaging of shark fin add to the product value. Overfishing due to increased demand has endangered many shark species. Mitigation measures are required to save the primitive species from becoming extinct. Identification of sharks based on fins, to track species being overfished has been a difficult task so far. However, recent developments on DNA based forensic techniques have made the problem somewhat easy. This method of identifying sharks from the fins earmarked for export, could serve well to implement control measures to this unscrupulous trade and save the stocks under depletion.
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Northern cod, comprising populations of Atlantic cod (Gadus morhua) off southern Labrador and eastern Newfoundland, supported major fisheries for hundreds of years. But in the late 1980s and early 1990s, northern cod underwent one of the worst collapses in the history of fisheries. The Canadian government closed the directed fishing for northern cod in July 1992, but even after a decade-long offshore moratorium, population sizes remain historically low. Here we show that, up until the moratorium, the life history of northern cod continually shifted towards maturation at earlier ages and smaller sizes. Because confounding effects of mortality changes and growth-mediated phenotypic plasticity are accounted for in our analyses, this finding strongly suggests fisheries-induced evolution of maturation patterns in the direction predicted by theory. We propose that fisheries managers could use the method described here as a tool to provide warning signals about changes in life history before more overt evidence of population decline becomes manifest.
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The age and growth of the shark Scoliodon laticaudus Muller and Henle [(= S. sonakowah (Cuvier) ] from Bombay waters have been studied by the length frequency method. The shark, which measures about 140 mm at birth, attains 260, 380,470, 530 and 590 mm at the end of 1, 2, 3, 4, and 5 years respectively after birth. There is no difference in the growth rates of males and females. The females grow to a larger size and to an older age than the males. The growth is more or less slow as in the case of other sharks. The von Bertalanffy growth equation has been fitted to the observed values, for which the parameters calculated were K (on an annual basis) = 0.2731, L o^ = 755.23 mm and to = • -.5664 years. The length-at-age thus calculated agreed fairly closely with the observed values. The maximum length of this shark in Bombay waters is about 660 mm. Over 75 per cent of the landings were in the age group of 2-4 years ranging from 380 to 530 mm in total length.
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The length-weight relationship of Scx)liodon laticaudus was calculated for males and females separately since regression coefficients in respect of sexes were significant. This shark prefers to feed on prawns during the premonsoon and monsoon periods and on fishes during the postmonsoon period. A change in the feeding habit of the shark is seen when it attains adolescence. Starvation of females above 350 mm during pregnancy is also observed. The spade nose shark, Scoliodon lati-caudus, one of the smallest tropical carchar-hinid sharks occupy mostly the shallow re-gions of the coastal waters. The earlier works on this species were mostly confined to the taxonomy and distribution. Literature on the biology of this shark is limited. A study on the description, bionomics and development of Scoliodon sorrakowah now considered a synonjmiof Scoliodon laticaudus was carried out by Setna and Sarangdhar (1948). The age and growth of Scoloiodon laticaudus were studied in Bombay waters by Nair (1976). Devadoss (1979) had carried out a detailed study on the maturity, breeding and develop-ment of this shark at Calicut. But there was no work on the other aspects of biology like length-weight relationship and food and feed-ing habits of this shark. Hence an attempt is made in this paper to fill up the gap.
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The first detailed elasmobranch fisheries data for the Kingdom of Bahrain are presented, based on surveys of fish markets in April 2012. At least 25 species were recorded, including undescribed taxa. The milk shark Rhizoprionodon acutus was the most frequently recorded species; together with the Arabian smoothhound Mustelus mosis and banded eagle ray Aetomylaeus nichofii, these species comprised 53% of individual abundance. Sharks were almost entirely small individuals <1 m total length (TL). Males of small shark species were largely mature, whereas nearly all individuals of larger sharks were immature. For several elasmobranch species, landings were significantly biased towards males, which were largely mature. The species assemblage showed some notable differences in composition to that of adjacent Qatar, sampled at the same time of year, highlighting the importance of local data collection.
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The first detailed information on the biological aspects of members of the economically important shark family Carcharhinidae in Indonesian waters is given. Carcharhinids dominated the landings of sharks at sites visited, comprising almost 60% of the total biomass of all sharks recorded, over an intensive 5 year sampling period. A total of 26 species of carcharhinids, representing half of the species in the Carcharhinidae, were recorded. The most abundant species, in terms of numbers recorded, were Scoliodon laticaudus and Carcharhinus falciformis, while the most abundant in terms of biomass were Prionace glauca, C. falciformis and Carcharhinus obscurus. Biological data are presented for 22 of the 26 species. The total length at which males attain maturity (L50) was determined for 10 species. The biological aspects of a number of species, most notably Carcharhinus sorrah and Loxodon macrorhinus, were found to differ from those recorded in the literature for other regions.
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This study presents a historical review, a meta-analysis, and recommendations for users about weight–length relationships, condition factors and relative weight equations. The historical review traces the developments of the respective concepts. The meta-analysis explores 3929 weight–length relationships of the type W = aLb for 1773 species of fishes. It shows that 82% of the variance in a plot of log a over b can be explained by allometric versus isometric growth patterns and by different body shapes of the respective species. Across species median b = 3.03 is significantly larger than 3.0, thus indicating a tendency towards slightly positive-allometric growth (increase in relative body thickness or plumpness) in most fishes. The expected range of 2.5 < b < 3.5 is confirmed. Mean estimates of b outside this range are often based on only one or two weight–length relationships per species. However, true cases of strong allometric growth do exist and three examples are given. Within species, a plot of log a vs b can be used to detect outliers in weight–length relationships. An equation to calculate mean condition factors from weight–length relationships is given as Kmean = 100aLb−3. Relative weight Wrm = 100W/(amLbm) can be used for comparing the condition of individuals across populations, where am is the geometric mean of a and bm is the mean of b across all available weight–length relationships for a given species. Twelve recommendations for proper use and presentation of weight–length relationships, condition factors and relative weight are given.
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Geographic variations in reproductive parameters of Mustelus manazo from five localities, four in Japan (Aomori, Tokyo Bay, Maizuru, and Shimonoseki) and one in Taiwan, were investigated from November 1994 to June 1996. Differences in age and length at sexual maturity from Aomori to Taiwan were approximately 3 years and 300mm in TL, respectively, for both sexes. The sharks appeared to mature at a later age and to a larger size in the most northern population, Aomori, and to be faster and smaller in the southern population, Taiwan. Mating, ovulation and fertilization periods were generally during May and August in the four localities of Tokyo Bay, Maizuru, Shimonoseki, and Taiwan. In Aomori, males showed no clear monthly variation in gonad index, and females in Aomori had a protracted reproductive period. Females became pregnant every year in all localities, except Aomori. The Aomori population apparently has a different reproductive cycle. The Tokyo Bay population is distinctive regarding growth and sex ratios of embryos. In all localities, the number of embryos per litter increased relative to total length of mother. There was no geographic variation in reproductive parameters between Maizuru and Shimonoseki. The coast between these last two sampling areas is a continuous steep slope with similar environmental conditions, and mixing of the two populations may occur.
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Age and growth were studied inRhizoprionodon taylori using specimens caught in Cleveland Bay, North Queensland, Australia. Von Bertalanffy growth parameters were estimated using three different techniques: vertebral ageing, back calculation and length frequency. Vertebrae from 138 specimens were sectioned and narrow circuli counted to estimate age. Marginal increment analysis verified that circuli were produced annually in late summer, probably as a result of stress during the mating season. The oldest female was 7 and male 6 years old. Von Bertalanffy growth parameters estimated from vertebral ageing data for males were tO = 0.410 yr, K = 1.337, L = 652.2 mm, and for females tO = 0.455 yr, K = 1.013 and L = 732.5 mm. Growth parameters determined by length frequency and back calculation techniques concurred with those from vertebral ageing. Growth of the 0+ age class was very rapid, averaging 140% of the size at birth in the first year. Males and females matured after only one year, the lowest age at maturity reported in the family Carcharhinidae. Annual growth increments decreased rapidly after maturity, and little growth occurred after three years.
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Placental viviparity has evolved inScoliodon taticaudus to a degree that rivals some eutherian mammals. Its eggs are the smallest known of any shark. They have a diameter of 1 mm, a dry weight of 0.0654 0.0100 mg and are nearly yolk-free. Implantation takes place at an early (3 mm) stage of development, and gestation is short (5–6 months). Comparison of the dry weight of the egg (0.065 mg) with the estimated dry weights of a mid-late term 90 mm embryo (910 mg) and a 152 mm neonate (3815.4 mg) reveals weight changes of 14219 and 58338 , respectively. Its normalized brood weight, a measure of maternal nutrient investment, is 49.5 g kg–1 female body weight for a six-month gestation. Comparisons with other species of placental and nonplacental sharks show thatS. laticaudus has a highly advanced form of matrotrophy. Maternal nutrients appear to be acquired by placental transport and by imbibition of uterine fluid. Hemotrophic placental nutrient transfer occurs across a unique uterine implantation site, termed the trophonematous cup, in which maternal blood appears to bathe the outer epithelium of the embryonic yolksac placenta. The latter is solid and filled with a three-dimensional network of capillaries and many free interstitial cells. The umbilical stalk contains the vitelline vessels but lacks a yolk duct. Its surface is amplified by many long, villous appendiculae, which consist of a vascular core that ramifies into a massive surface capillary network invested by a simple squamous epithelium. The appendiculae ofS. laticaudus most likely are sites of gas exchange and possibly the uptake of small molecules. They are unlike the appendiculae described in any other placental shark and exhibit design principles similar to those of the uterine trophonemata of matrotrophic rays.
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This paper presents data from the first major survey of the diversity, biology and fisheries of elasmobranchs in the Persian (Arabian) Gulf. Substantial landings of elasmobranchs, usually as gillnet by-catch, were recorded in Kuwait, Qatar and the Emirate of Abu Dhabi (part of the United Arab Emirates), although larger elasmobranchs from targeted line fisheries were landed in Abu Dhabi. The elasmobranch fauna recorded was distinctive and included species that are undescribed, rare and have a highly restricted known distribution. Numerical abundance was dominated by sharks (c. 80%), of which carcharhinids were by far the most important. The milk shark Rhizoprionodon acutus and whitecheek shark Carcharhinus dussumieri together comprised just under half of all recorded individuals. Around 90% of recorded sharks were small (50-90 cm total length, L(T) ) individuals, most of which were mature individuals of species with a small maximum size (<100 cm L(T) ), although immature individuals of larger species (e.g. Carcharhinus sorrah and other Carcharhinus spp.) were also important. The most frequently recorded batoid taxa were cownose rays Rhinoptera spp., an undescribed whipray, and the granulated guitarfish Rhinobatos granulatus. The first size, sex and maturity data for a wide range of Gulf elasmobranch species are presented (including L(T) at 50% maturity for males of four shark species) and include some notable differences from other locations in the Indo-West Pacific Ocean. A number of concerns regarding the sustainability of the fishery were highlighted by this study, notably that most of the batoid species recorded are classed by the IUCN Red List as vulnerable, endangered, data deficient or not evaluated. Despite their considerable elasmobranch landings, none of the three countries sampled have developed a 'Shark Plan' as encouraged to do so under the FAO International Plan of Action: Sharks. Furthermore, Kuwait and Qatar currently report zero or no elasmobranch landings to the FAO.
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Many species of chondrichthyans (sharks, rays and chimaeras) reproduce throughout the year, lacking a distinct seasonality in the timing of their reproduction with young born throughout the year. Since age determination in fish usually relies on counting regularly deposited growth increments on hard body parts (e.g. vertebrae, spines, otoliths) the age of an aseasonally reproducing animal after the first growth increment is formed is unknown. We explored the implications of this for the estimation of growth and maturity parameters using the milk shark, Rhizoprionodon acutus, a small, tropical, shark that reproduces aseasonally off northern Australia. Since R. acutus grows rapidly after birth, not accounting for an aseasonal reproductive cycle led to unrealistic projections of growth for this species. We describe a simple method for adjusting individual ages that improves projections of growth and reduces the level of error around growth parameter estimates. This is compared with growth estimates when ages are left unadjusted or adjusted to a mean value to account for aseasonal reproduction. Using back calculated length-at-age data, model-averaged growth across five models gave an asymptotic size (L∞) of 859 mm for females and 821 mm for males. Standard von Bertalanffy growth parameters were L∞ = 861 mm, K = 0.63, L0 = 423 mm for females and L∞ = 821 mm, K = 0.94, L0 = 424 mm for males. The oldest female and male were 8.1 and 4.5 years old, and the largest female and male measured were 940 and 931 mm. The size at which 50% of females and males were mature was 780 and 742 mm. The age at which 50% of females and males were mature was 1.8 and 1.1 years of age. Incorporating the effects of an aseasonal reproductive cycle into growth analysis is an important step for maximising the accuracy of age and growth results and is especially important for small, tropical species such as R. acutus that complete much of their growth in the first year of life.
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Concern has been raised that the widely used spawning stock biomass might not be a sensitive index of the reproductive potential of fish populations. On the other hand, there is increasing evidence that fishing affects life history traits of fish. Fish compensate for fishing mortality by faster growth, earlier maturity and increased fecundity. Because life history traits are affected by fishing and are also critical in determining the population growth rate, they may be used as indices of population viability. In this paper, the traits that meet both criteria are investigated among age-at-maturity, fecundity and growth. In the framework of matrix population models, the life table response experiments (LTRE) method is used to quantify the effects of fishing on demographic parameters, and the contributions of these effects to the population growth rate. This approach is used for comparing parameters and population growth rates of flatfish populations between periods with different fishing pressure, it is found that age-at-maturity, the proportion of repeat spawners in the spawning stock and some proxy for lifetime fecundity may be used as indices of population viability.
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It is generally assumed that fish populations are regulated primarily in the juvenile (pre-recruit) phase of the life cycle, although density dependence in growth and reproductive parameters within the recruited phase has been widely reported. Here we present evidence to suggest that density-dependent growth in the recruited phase is a key process in the regulation of many fish populations. We analyse 16 fish populations with long-term records of size-at-age and biomass data, and detect significant density-dependent growth in nine. Among-population comparisons show a close, inverse relationship between the estimated decline in asymptotic length per unit biomass density, and the long-term average biomass density of populations. A simple population model demonstrates that regulation by density-dependent growth alone is sufficient to generate the observed relationship. Density-dependent growth should be accounted for in fisheries' assessments, and the empirical relationship established here can provide indicative estimates of the density-dependent growth parameter where population-specific data are lacking.
The First Indian Fisheries Forum
  • K. K. Appukuttan
  • K. P. Nair
Marine Fisheries Policy Series No. 2
  • S. J. Kizhakudan
  • P. U. Zacharia
  • S. Thomas
  • E. Vivekanandan
  • M. Muktha