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Reproductive biology of the black marlin, Istiompax indica, off southwestern and eastern Taiwan
Abstract
The reproductive biology of the black marlin, Istiompax indica, was assessed by examining 4762 fish (458 ovary samples) caught by offshore longliners off southwestern and eastern Taiwan during 2004–2007. The overall sex ratio (0.67 off southwestern and 0.86 off eastern Taiwan) was significantly different from 0.5 (p < 0.01), with females predominant in both sampling areas. Monthly variation in the gonadosomatic index, mean monthly oocyte diameter, and histological analysis indicated that black marlin spawned off southwestern Taiwan during March and April. Although black marlin aggregated for feeding purposes off eastern Taiwan, no evidence of spawning was found in samples from this location. The size-at-50% maturity was estimated at ∼195 cm eye-to-fork-length (EFL) (∼5.4 years old) for female black marlin. Fully mature female black marlin were estimated to spawn once every ∼2.8 days using the hydrated oocyte method. Batch fecundity was estimated from 19 females, and varied 10-fold from 3.2 to 32 million eggs, with an average of 15.2 ± 1.9 (±S.E.) million eggs. The average relative batch fecundity was estimated at 105.7 ± 12.0 (±S.E.) eggs per gram of body weight. The relationships between batch fecundity (BF, million eggs), EFL and round weight (i.e., whole fish weight; RW, kg) were BF = 2 × 10−12EFL8.05 (r2 = 0.43) and BF = 8 × 10−4RW1.95 (r2 = 0.37), respectively. The parameters estimated in this study are key information for stock assessments of black marlin in the western Pacific Ocean and will contribute to the management, conservation and sustainable yield of this species
... Black marlin (Istiompax indica) is a highly migratory species that is widely distributed in tropical and subtropical waters [74,[122][123][124]. As in other billfish, the distribution of black marlin varies seasonally, with high densities in summer (in high latitudes) and in winter (in low latitudes) [74,125]. ...
... Black marlin often occurs in relatively high numbers in the East China Sea, northwest Coral Sea, Arafura Sea, Sulu Sea, Celebes Sea, around Taiwan, in northwestern Australia and off the coast of Panama [125,126]. Their migrations are correlated with SSTs of 25-28 • C [124,127]. It seems that this and other billfish species tend to visit continental margins and seamounts, where they are more exposed to human activities [128,129]. ...
... The trans-equatorial and trans-Pacific movements of black marlin [130], together with the fact that they do not form spawning aggregations [124], hinder the acquisition of knowledge about the distribution of this species. More detail-oriented research (temporal and spatial) is necessary to define the boundaries of spawning habitats [125]. ...
Migrations of large pelagic fishes across the Pacific are usually inferred from tagging or genetic studies. Even though these techniques have improved over time, they still fail to demonstrate large transoceanic migrations, usually proposing ‘routes’ that do not cycle seasonally. The current study uses the concept of ‘philopatry’ in 11 large pelagic fish species, i.e., the tendency for animals to return to their natal site to reproduce. Tentative migration routes and maps emerge by applying this concept to the movements extracted through a comprehensive review of the literature on satellite and conventional tagging, and population and subpopulation linkages inferred from genetic and/or genomic studies. Moreover, when comparing these proposed migration routes and the mapped reconstructed catch (1950–2016, Sea Around Us) of each species in the Pacific, similarities emerge, reinforcing the accuracy of these migration cycles informed by philopatry. Finally, by superposing the migration routes of our 11 species, we identified areas of the Pacific that are part of the inferred migration routes of multiple species, leading to a discussion of possible ‘blue corridors’ that would protect the studied species’ key migration routes and stocks, which are important for the fisheries, culture and nutrition of Pacific islanders.
... Tissue samples from I. indica were collected from 12 different locations from throughout the Indian and Pacific Oceans between 1985 and 2016 ( Temporal replicates from the region of known spawning areas off eastern Australia and Taiwan were obtained to quantify any changes in allele frequencies through time (Domeier and Speare, 2012;Sun et al., 2015). Each temporal replicate was collected approximately one decade apart with a decade, with each decade representing approximately 2.5 generations of change (Speare, 2003;Sun et al., 2015). ...
... Tissue samples from I. indica were collected from 12 different locations from throughout the Indian and Pacific Oceans between 1985 and 2016 ( Temporal replicates from the region of known spawning areas off eastern Australia and Taiwan were obtained to quantify any changes in allele frequencies through time (Domeier and Speare, 2012;Sun et al., 2015). Each temporal replicate was collected approximately one decade apart with a decade, with each decade representing approximately 2.5 generations of change (Speare, 2003;Sun et al., 2015). For both sites temporal replicates spanned up to fourdecades (up to 10 generations). ...
... The four decades of samples which were analysed account for approximately 10 generations of change (Domeier and Speare, 2012;Sun et al., 2015). The absence of temporal changes in allele frequencies among collection locations provides support that the genetic population structure identified herein is a long-term, stable feature throughout the geographic range of I. indica. ...
The black marlin [Istiompax indica (Morrow, 1968)] is one of the largest teleost fish and has a continuous distribution throughout tropical waters of the Indo-Pacific. Throughout its range, I. indica has been documented to undertake wide-ranging, transoceanic and trans-equatorial movements. In coastal waters and on the high seas I. indica may be targeted by commercial and artisanal fisheries, but the majority of the world’s landings are as a by-product species of tuna longline and purse seine fisheries. Due to its large size and ‘fighting qualities’, I. indica is also valued as a prized sport or game fish in recreational fisheries. While considerable effort to understand the level of exploitation of high value species such as the tunas has occurred, the stock status for many other species caught on the high seas such as many sharks, dolphinfish and istiophorid billfishes remain unassessed. This lack of attention is reflected by I. indica being listed as ‘Data Deficient’ on the International Union for Conservation of Nature Red List. The lack of attention and data available on I. indica extends to biological, ecological, taxonomic and fisheries catch records, which in turn hinders stock assessments and management. This thesis aims to improve the biological and ecological knowledge of I. indica, thereby providing a stronger foundation for assessments and management of stocks globally.
Since the original taxonomic classification of billfishes (suborder Xiphioidei) using morphological data, the systematics of billfishes have been reshaped using genetic markers. The results of phylogenetic analysis recommended the expansion of Istiophoidae from three genera to five; Makaira, Istiophorus, Kajikia, Istiompax and Tetrapturus. Despite the taxonomic revision, several aspects of the Istiophoridae phylogeny remained unresolved, including the position of the genus Istiompax. To address the outstanding issues within Istiophoridae, a phylogenetic analysis was undertaken using whole mitogenomes for all billfish species. Phylogenetic analyses supported the presence of five genera within Istiophoridae and resolved the placement of Istiompax as the sister taxon to Kajikia + Tetrapturus. The investigation also provided new insights into the close genetic relatedness between a number of morphologically similar species within Tetrapturus and Kajikia. This study increased the phylogenetic resolution of istiophorid billfishes and highlighted the need for further work to resolve the genetic relationships among closely related billfish.
Increases in landings of billfishes has highlighted the need for accurate reporting and identification of morphologically similar species by boats operating in multi-species tuna fisheries. In the Atlantic Ocean, high rates of misidentification and misreporting between species of billfish have been identified, confounding our understanding of the stock status of Atlantic billfishes. Genetic analysis was undertaken to investigate whether misidentification of Pacific billfishes by fisheries observers was occurring in multi-species tuna fisheries. Tissue samples were collected from billfish that fisheries observers morphologically identified as I. indica. Genetic analysis of 83 samples using a suite of microsatellite markers revealed that 77.1% of the putative I. indica were in fact blue marlin (Makaira nigricans) and 2.4% were striped marlin (Kajikia audax). The high rate of misidentification (~80%) by observers places considerable uncertainty over historic catch ratios of Indo-Pacific marlin species and stock assessments relying on the validity of these data.
Increasing fishing pressure and uncertainty surrounding the valid identification of I. indica landed by commercial operators provided the impetus for development of methods for collecting tissue samples for population genetics research from recreational fisheries. A pilot investigation was undertaken to engage the Australian game fishing community and promote the non-lethal collection of tissue samples from I. indica. After validating the success of a pilot minimally invasive, non-lethal sampling program, the program was extended throughout the range of I. indica. Through a combination of non-lethal recreational sampling and tissue collected from commercially landed I. indica at fish markets, a total of 465 samples were collected from 13 locations throughout the species’ range. A suite of 17 microsatellite markers and 1557 nuclear single nucleotide polymorphisms was used to investigate genetic stock structure throughout the Indo-Pacific. The results indicated the presence of three genetic stocks 1) Indian Ocean, 2) South Pacific Ocean and 3) North-west Pacific Ocean. Resolution of the genetic stock structure of I. indica provides a basis for implementing biologically meaningful assessments and management of the species throughout its range. The development of additional biological and ecological information will continue to reduce uncertainty around the status of black marlin stocks, and provide decision makers with capacity to drive the enforcement of other fishery-specific management controls.
Understanding how environmental factors influence the distribution and abundance of billfishes provides insights into how environmental change may alter migration phenology, spawning, vertical distribution and survival rate of larvae. To characterise the vertical habitat use of I. indica, temperature-depth profiles from 102 pop-up satellite archival tags deployed on I. indica off the east coast of Australia were examined in this study. Modelling of environmental variables revealed location, sea surface height deviation, mixed layer depth, and dissolved oxygen to all be significant predictors of vertical habitat use. This investigation also demonstrates, for the first-time, ontogenetic differences in vertical habitat in a species of billfish. Distinct differences in diel movements, diving behaviour and thermal range were observed among different size classes of I. indica. The differences in the diving behaviour among size classes were suggested to reflect ontogenetic differences in foraging behaviour and/or physiology. The outcomes from this study characterise the vertical habitat use of I. indica and further the understanding of pelagic fish ecophysiology in the presence of global environmental change. Furthermore, the vertical distribution of I. indica identified herein provides information that could be used by policy makers to reduce bycatch mortality rates in fisheries where they are not targeted.
It is anticipated that the outcomes of this research will provide fisheries managers with an enhanced understanding of population biology and ecology that can be used to inform management decisions and future research priorities of billfish. Knowledge gained by this thesis also add new insights into phylogeny and physiology of large pelagic fishes to benefit the disciplines of conservation and systematics.
... The black marlin, Istiompax indica is one of the world's largest teleosts (females up to 700 kg) and individuals form seasonal aggregations to spawn and forage. Spawning aggregations of I. indica are documented to occur off northeastern Australia from September to November (Leis et al. 1987;Domeier and Speare 2012) and south-western Taiwan in March and April (Nakamura 1975;Sun et al. 2015). Coinciding with these aggregations is increased commercial and recreational fishing pressure (Domeier and Speare 2012). ...
... However, the results of our study clearly show the presence of three genetically discrete contemporary populations of I. indica in the central Indo-Pacific based on nuclear markers. The geographic separation of the south-western Pacific Ocean, eastern Indian Ocean and South China Sea populations presumably correspond to regions that contain discrete spawning grounds, given what is known of the biology of the species (Domeier and Speare 2012;Sun et al. 2015). In light of the large dispersal capability of I. indica, as demonstrated through numerous inter-equatorial and intra-ocean movements (Ortiz et al. 2003), our results suggest that reproductive philopatry may be the primary driver of the broad-scale patterns of spatial genetic structure observed in the nuclear genome. ...
... Unlike other large pelagic fishes which adhere to confluent or multiple aggregation spawning concepts (Kopf et al. 2012), the biology of I. indica suggests that each sub-population is philopatric to a single, geographically disparate spawning ground. Biological differences between I. indica from the South China and Coral Sea including larger average size and older gravid fish in the Coral Sea (Speare 2003;Sun et al. 2015) providing further evidence to justify separate genetic stocks. However, biological differences may also be reflective of local fishing pressure, with the South China Sea open to heavy commercial fishing pressure whereas the Coral Sea fishery operating on a recreational-only catch and release basis. ...
The black marlin Istiompax indica is a highly migratory species and as a result is expected to show little genetic population structure throughout its broad geographic range. Tissue samples from 183 I. indica were collected from three geographic regions within the central Indo-Pacific and analysed using mitochondrial and nuclear DNA markers. Nuclear genetic heterogeneity was found among populations in the south-western Pacific Ocean, eastern Indian Ocean and South China Sea (significant FST values of 0.013–0.037). Combining information from nuclear markers with published movement and reproductive data suggests that reproductive philopatry plays a role in maintaining contemporary I. indica population structure. Analyses of the mitochondrial control region did not reflect this pattern; however, it identified historical population structure. Differing patterns of genetic population structure revealed by mitochondrial and nuclear markers demonstrated that a transition must have occurred between historica
... The results of this study indicated that females with larger body sizes are more likely to have an extended spawning season than females with smaller body sizes. Furthermore, previous studies reported that batch fecundity tends to be related to female body sizes, with larger females of several species of billfishes having higher batch fecundity (Chiang et al., 2006a;Sun et al., 2009;Sun et al., 2015). Although there were only nine samples available for spawning frequency analysis in this study, which was not considered to be sufficient for estimating spawning frequency by body length classes, it seems reasonable to assume that spawning frequency may also relate to female body size, i.e. larger females are likely to have a shorter spawning interval than smaller females. ...
... Nakamura (1949) indicated that the fecundities of billfishes ranged between 1 and 1·2 million eggs and varied by species and size. Compared with M. nigricans and I. indica, in the Pacific Ocean, the body of K. audax is more laterally compressed, which might limit the body cavity in accommodating larger gonads and batch fecundity might be highly correlated with body size (Chiang et al., 2006a;Sun et al., 2009Sun et al., , 2015. Sample sizes obtained from previous studies however and the present report were probably too small to describe the relationship between batch fecundity and body size for K. audax with great precision. ...
Length and mass data for 1260 (536 females, 683 males, 41 sex unknown) striped marlin Kajikia audax were collected at the fish markets of Tungkang, Singkang and Nanfangao from July 2004 to September 2010. Of these samples, 534 gonads (236 females and 298 males) ranging from 95 to 206 cm in eye-to-fork length (LEF) and 8 to 88 kg in round mass (MR), were collected. Chi-square tests indicated sex ratios were homogeneous among months in 2004 and 2006–2008, but not in 2005, 2009 and 2010; and there were significant differences in sex ratio by size. The overall sex ratio (RS) differed significantly from the expected 0·5. Kajikia audax are sexually dimorphic and the proportions of females increased with size between 140 and 210 cm LEF. Reproductive activity was assessed using a gonado-somatic index (IG), external appearance of the gonads and histological examination and results indicated that the spawning season occurred from April to August with a peak in June to July. Based on histological observations and the distribution of oocyte diameters, K. audax are multiple spawners and their oocytes develop asynchronously. The estimated length-at-50% maturity (LEF50) was c. 181 cm (c. 4·8 years of age) for females. The proportion of reproductively active females in the spawning season with ovaries containing postovulatory follicles (0·27) indicated that they spawned every 3·7 days on average. The hydrated oocyte method estimated mean ± S.D. batch fecundity (FB) to be 4·4 ± 2·02 million eggs; average relative fecundity was 53·6 ± 13·9 oocytes g⁻¹MR; and the average annual fecundity was 181·3 ± 48·3 million eggs. The parameters estimated in this study are key information for stock assessments of K. audax in the north-western and central Pacific and will contribute to the conservation, management and sustainable yield of this species.
... The developmental stages of oocytes were classified according to Yamamoto (1956) and Granada et al. (2004) with a few modification for this species, and the histological classification of atretic oocytes and post-ovulatory follicles followed Hunter and Macewicz (1985). Ovarian maturity stages were estimated from the appearance of the most advanced oocytes, post-ovulatory follicles and atretic oocytes (Granada et al. 2004;Alejo-Plata et al. 2011;Sun et al. 2015;Okochi et al. 2016). ...
Ovarian maturation, size at sexual maturity and the annual reproductive cycle of female crimson sea bream Evynnis tumifrons were studied using samples collected from April 2012 to June 2014 off the southwestern coast of Kyushu, Japan. A total of 801 ovaries were examined histologically to estimate the degree of ovarian maturation. E. tumifrons showed an asynchro-nous ovary signifying multiple spawning in a single reproductive season. Ovarian maturity stages were classified into six categories based on the appearance of the most advanced oocytes, post-ovulatory follicles and atretic oocytes in the ovary as follows: immature, maturing, mature, spawned, spent and resting. Females with ovaries at maturing, mature, spawned or spent stages were defined as sexually mature individuals, and the size at sexual maturity was estimated to be 179-mm fork length based on 50% maturity size. Monthly changes in the gonadosomatic index and the occurrence of mature or spawned maturity stages showed that the spawning season lasts from November to May with an intermission in March 2013. The factor responsible for this intermission was considered to be the low water temperature that occurred in the preceding month.
... The developmental stages of oocytes were classified according to Yamamoto (1956) and Granada et al. (2004) with a few modification for this species, and the histological classification of atretic oocytes and post-ovulatory follicles followed Hunter and Macewicz (1985). Ovarian maturity stages were estimated from the appearance of the most advanced oocytes, post-ovulatory follicles and atretic oocytes (Granada et al. 2004;Alejo-Plata et al. 2011;Sun et al. 2015;Okochi et al. 2016). ...
Age and growth of crimson sea bream Evynnis tumifrons were studied using samples collected from October 1999 to August 2013 off the southwestern coast of Kyushu, Japan. Ring marks (outer edges of opaque zones) on the 1805 transversely sectioned otoliths were counted and seasonality in their deposition was validated by marginal increment. Our results revealed that one ring mark was formed per year from late spring to early summer. Assuming December as the birth month, ages were assigned to every individual according to the number of ring marks and the value of marginal increments. Growth was estimated by fitting the von Bertalanffy growth function to the length-at-age and weight-at-age data. The estimated growth curves did not differ significantly between the sexes, and the growth curve of the pooled data was \(L_{\text{t}} = 271\left( {1 - { \exp }\left( { - 0.604\left( {t + 0.193} \right)} \right)} \right)\) for length-at-age and \(W_{\text{t}} = 519\left( {1 - { \exp }\left( { - 0.484\left( {t + 0.625} \right)} \right)} \right)^{3}\) for weight-at-age. The maximum age observed was 15 years for females and 16 years for males.
... The developmental stages of oocytes were classified according to Yamamoto (1956) and Granada et al. (2004) with a few modification for this species, and the histological classification of atretic oocytes and post-ovulatory follicles followed Hunter and Macewicz (1985). Ovarian maturity stages were estimated from the appearance of the most advanced oocytes, post-ovulatory follicles and atretic oocytes (Granada et al. 2004;Alejo-Plata et al. 2011;Sun et al. 2015;Okochi et al. 2016). ...
... Values in bold are estimated (see methods for details). *estimate for female., 2012), 5 (Torres Jr., 1991; Wu et al., 2001; Chang et al., 2013), 6 (Observers Data base), 7 (Observers Data base), 8 (Observers Data base), 9 (Grant et al., 1978; Torres Jr, 1991; Paul, 1992; Kailola et al., 1993; Annala, 1994), 10 (Nakamura, 1985; Cyr et al., 1990; Speare, 2003 Speare, , 2007 Romanov and Romanova, 2012; Sun et al., 2015), 11 (Nakamura, 1985; Chiang et al., 2004 Chiang et al., , 2006a Romanov and Romanova, 2012), 12(Nakamura, 1985; Kailola et al., 1993; Kopf et al., 2011; Romanov and Romanova, 2012), 13 (Nakamura, 1985; Cyr et al., 1990; Shimose et al., 2008; Sun et al., 2009; Romanov and Romanova, 2012), 14 (Potier et al., 2011), 15 (Gon, 1990), 16 (Al-Baz et al., 1999), 17 (Zischke et al., 2013; Observers Data base), 18 (Landau, 1965; Muthiah, 1985, 1990; Collette, 2010), 44 (Claro, 1994), 45 (Claro, 1994), 46 (Brown-Petersen et al., 2000; McBride et al., 2008; Viana et al., 2013), 47 (Bök and Oray, 2001; Macías et al., 2005a; Palandri et al., 2009), 48 (Grudtsev and Korolevich, 1986; Cayré et al., 1993), 49 (Rodriguez-Roda, 1979; Diouf, 1980; Collette and Nauen, 1983; Collette et al., 2011c), 50(Batts, 1972; Cayré and Farrugio, 1986; Vilela and Castello, 1993; Andrade and Campos, 2002; Andrade et al., 2004, Castello, 2007), 51 (Castello and Gagliardi, 1969; Postel, 1955; Hansen, 1989; Macías et al., 2005b; Zaboukas and Megalofonou, 2007), 52 (Lima et al., 2007; Nóbrega and Lessa, 2009a,b; Chellappa et al., 2010), 53 (Finucane et al., 1986; Lessa et al., 2009a; Nóbrega and Lessa, 2009c), 54 (Finucane and Collins, 1984; Juan-Jordá et al., 2013b), 55 (Bard, 1981; Lee and Yeh, 2007; ICCAT, 2008 ICCAT, , 2011 ICCAT, , 2014), 56 (Hazin, 1993; Arocha et al., 2001; Lessa and Duarte-Neto, 2004; IGFA, 2010), 57 (Doray et al., 2004; Freire, 2009; IGFA, 2010; Bezerra et al., 2013), 58 (Parks et al., 1982; Duarte-Neto et al., 2012; Figueiredo, 2007 higher values of k when compared to target species (T), lower values of L 50 when compared to BY/KC and smaller values of L max than those categorized as T and BY/KC (P < 0.05,Fig. 4). ...
The black marlin Istiompax indica is an apex marine predator and is susceptible to overfishing. The movement ecology of the species remains poorly-known, particularly within the Indian Ocean, which has hampered assessment of their conservation status and fisheries management requirements. Here, we used pop-up archival satellite tags to track I. indica movement and examine their dispersal. Forty-nine tags were deployed off Kenya during both the north-east (November–April) and south-west (August–September) monsoon seasons, providing locations from every month of the year. Individual I. indica were highly-mobile and track distance correlated with the duration of tag attachment. Mean track duration was 38 days and mean track distance was >1,800 km. Individuals dispersed in several directions: north-east into Somalian waters, and up to northern Oman; east towards the Seychelles; and south into the Mozambique Channel. Their core habitat shifted seasonally, and overlapped with areas of high productivity off Kenya, Somalia, and Oman during the first half of the year. A second annual aggregation off the Kenyan coast, during August and September, did not coincide with high chlorophyll-a concentrations or thermal fronts, and the drivers of the species’ distribution in this second aggregation was unclear. We tested their habitat preferences by comparing environmental conditions at track locations to those at locations along simulated tracks based on the real tracks. Observed I. indica preferred cooler water with higher chl-a concentrations and stayed closer to the coast than simulated tracks. The rapid and extensive dispersal of I. indica from Kenya suggests that there is likely a single stock in the Western Indian Ocean, with individuals swimming between areas of high commercial catches off northern Somalia and Oman, and artisanal and recreational fisheries catches throughout East Africa and Mozambique.
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Ovarian maturation, size at sexual maturity and annual reproductive cycle of female crimson sea bream Evynnis tumifrons were studied using the samples collected from April 2012 to June 2014 off the southwestern coast of Kyushu, Japan. A total of 801 ovaries were examined by histological observation to estimate the degree of ovarian maturation. E. tumifrons showed an asynchronous ovary signifying multiple spawning in a single reproductive season. Ovarian maturity stages were classified into six categories based on the appearance of the most advanced oocytes, postovulatory follicles and atretic oocytes in the ovary as follows: immature, maturing, mature, spawned, spent and resting. Females with the ovaries at maturing, mature, spawned or spent stages were defined as sexually mature individuals, and the size at sexual maturity was estimated to be 179 mm fork length based on 50% maturity size. Monthly changes in gonadosomatic index and the occurrence of mature or spawned maturity stages showed that spawning season last from November to May with an intermission in March 2013. The mechanism responsible for this intermission was considered to be the low water temperature that occurred in the preceding month.
The spawning characteristics of round herring Etrumeus teres remain unclear despite its commercial and ecological importance. The oocyte growth and maturation during the spawning cycle of round herring were examined based on the samples collected at different hours of the day off southern Japan during their 2015 spawning season. The ovarian histological analysis showed that germinal vesicle migration (GVM) and subsequent germinal vesicle breakdown occurred from the afternoon to the next morning. The appearance of hydrated oocytes indicated that round herring spawned between midday and evening. Degeneration of post-ovulatory follicles and the ovarian developmental stage suggested that individual round herring did not spawn on consecutive days but rather spawned at an interval of at least 2 days. The oocyte diameters of the second clutch (subsequent spawning batch) increased gradually when the first clutch (spawning batch) underwent GVM and hydrated, but oocytes in the second clutch increased significantly in size immediately after spawning, suggesting that vitellogenesis of the second clutch may be accelerated for the next spawning. Size at maturity in all females was estimated to 17.6 cm in body length (BL), and batch fecundity was 1409–21,023 oocytes (15.7–24.0 cm in BL). Our findings provide a foundation for conducting stock assessment studies on round herring using an egg production method.
Half of anchovy females reach sexual maturity by 96mm standard length (95% confidence level 94-97mm). Spawning batch fecundity was an exponential function of female weight. In the Los Angeles Bight the proportion of the population spawning/day declined from 13.4% of the females in January-February to 10.0% in March-April and fell to 3.4% by June. Spawning of anchovy in Monterey Bay had nearly ended in March, and 71.9% of the females were classed as post-spawning. By sunset eggs were completely hydrated and spawning began; spawning reached a maximum between 2200-2300 hours and ended by 0200 hours. At 0600 hours females destined to spawn the next night began to hydrate eggs.-from Sport Fishery Abstracts
Histological criteria to age postowlatory follicles were developed from examination of laboratory-spawned skipjack tuna; the criteria were used to estimate the frequency of spawning of skipjack tuna from the South Pacific. Examination of 87 skipjack tuna from field collections taken in October-November indicated that spawning occurred nearly every day. The fraction of mature females with postovulatory follicles,
Lower jaw fork length (LJFL) was measured in 551 female and 386 male swordfish at Tungkang, Nanfangao, and Shinkang fish markets during September 1997 to July 2000, and ovaries were removed from 208 of the swordfish collected at the Shinkang fish market during July 1998 to June 2000 whose LJFLs ranged between 95 and 257 cm. The sex ratio (the proportion of females to the total number of females and males) increased as the LJFL increased beyond 150 cm (sex ratio = 1 x 10-4 LJFL1.6601), and all fish with LJFLs of greater than 210 cm were females. The estimated mean body length at sexual maturity (L50) for females was 168.2 cm; the smallest mature female was 135 cm. Among mature females, 79% were in the developmental stage of ripening and 21% were in the resting stage; no individual was in the spawning or recently spawned stage. No hydrated oocytes were observed in any of the female gonad samples. These observations indicate that swordfish do not spawn in the waters of Taiwan. According to the relationship between developmental stage and the diameters of oocytes, 50% of individuals will be reproductively active at an oocyte diameter of 372.1 μ which can be used as an index for reproductive activity of swordfish in the waters of Taiwan.
Appraisal of reproductive strategy and fecundity is necessary to evaluate the reproductive potential of individual fish species. To estimate reproductive potential, one needs to consider a variety of attributes including onset of maturity, fecundity, atresia, duration of reproductive season, daily spawning behaviour and spawning fraction. In this contribution, we review several methods currently used to estimate fecundity of marine fishes collected in the field in relation to their reproductive strategy. The advantages and disadvantages of each method are provided. Requirements are given to appropriately sample gonadal tissue that will enable researchers to establish incidence of sexual maturity and estimate fecundity.
The sailfish (Istiophorus platypterus) (Istiophoridae) is widely distributed in tropical and temperate oceans of the world. In Taiwan, sailfish are abundant from Apr. to Oct. (with the peak abundance from May to July) and substantially contribute to the economy of the east coast of Taiwan. In this study, we estimated the spawning frequency and batch fecundity of female sailfish off eastern Taiwan based on histological examinations of the ovaries of 246 sailfish collected at the Shinkang Fish Market in eastern Taiwan between Jan. 1999 and Sept. 2000. The proportion of mature females with ovaries containing postovulatory follicles was 0.53 on average during the spawning season of Apr. to Sept., suggesting that on average, the sailfish spawns every 1.89 d. Sailfish are multiple spawners with asynchronous oocyte development. Batch fecundity for 18 females with the presence of hydrated oocytes but without postovulatory follicles yielded (0.2-2.48) × 106 eggs with an average of 1.3 × 106 eggs. The relationships between batch fecundity in millions (BF) versus round weight (RW, in kg) and lower jaw fork length (LJFL, in cm) are BF = 0.0944 x RW - 1.3134 (r2 = 0.724) and BF = 5.46 × 10-5 LJFL6.283 (r2 = 0.667), respectively. http://zoolstud.sinica.edu.tw/Journals/45.4/483.pdf.
The average female Scombev juponicus collected from April through July of 1985 spawned 8.8 times during the 101-day sampling period. The average interval between spawns was 1.3 days in 32 females with more than one spawning stage in their ovary. The average batch fecundity was 68,400 oocytes, or 168 oocytes per gram female wet weight (without ovary).
This contribution describes and identifies the most common reproductive strategies of a large number of commercially important fish species of the North Atlantic with regard to oocyte development, ovary organization, recruitment of oocytes and spawning pattern. Group-synchronous ovary organization, determinate fecundity and batch spawning was the most common suite of associated reproductive traits observed among North Atlantic fishes (e.g., gadoids, pleuronectoids). Another common type of female reproductive strategy among these species was synchronous, determinate and total spawning which occurred in a number of semelparous (eels, Anguilla sp., capelin, Mallotus villosus) and iteroparous species (e.g., redfishes, Sebastes sp., monkfishes, Lophius sp., herring, Clupea harengus, and elasmobranchs). Asynchronous, indeterminate and batch spawning occurred among anchovies, Engraulis sp., European hake, Merluccius merluccius, mackerels, Scomber sp. and Trachurus sp., swordfish, Xiphias gladius, and others. Categorization of species according to reproductive strategy assists in the estimation of species-specific fecundity and reproductive potential using various developed protocols.
The western stock of Atlantic mackerel (Scomber scombrus) is an important fishery resource in European waters with a catch potential of about 500,000 tonnes. Since 1977 the spawning stock size has been estimated every 3 years using the annual egg production method (=total fecundity method). In 1989 the daily egg production method (=batch fecundity method) was applied for the first time. Daily egg production in the middle of the spawning season was estimated from a subset of the data collected for the regular annual production survey. Fish were captured by trawl for measurement of batch fecundity and daily spawning fraction. The daily egg production method gave a biomass estimate 15% higher than the annual egg production method. This could be explained by a loss of eggs from the potential fecundity during the course of the season. The daily egg production method overcomes the problem of indeterminate fecundity in this species and requires only one, instead of five plankton surveys. Suggested improvements to the annual production method to measure changes in annual fecundity incur penalties in ship-time requirement. It is concluded that in the future, on the grounds of improved precision and lower cost, the daily egg production method will be preferred for estimation of biomass in this species.
Istiophorid billfishes are highly migratory species that inhabit the tropical and subtropical, epipelagic waters of the world's oceans, a large, relatively homogeneous environment that lacks significant physical barriers. Based on these observations alone, one would not expect marlins, sailfish and spearfishes to exhibit substantial stock structure. This assumption has been evaluated with a variety of techniques, including analyses of morphological characters, adult distribution, tag and recapture data, the spatial and temporal distribution of spawning and, recently, molecular genetic characters. This paper focuses on inferences of istiophorid billfish stock structure derived from investigation of several different classes of molecular markers, and reviews our current understanding of the genetic basis of stock structure of striped marlin, white marlin, blue marlin, sailfish and black marlin. Significant genetic differences exist between Atlantic and Indo-Pacific populations of blue marlin and sailfish, and the presence of distinct mitochondrial DNA lineages suggests that ocean populations were isolated in the past. However, the occur-rence of identical genotypes in both oceans is evidence of recent genetic contact. The genetic data do not support recognition of separate Atlantic and Indo-Pacific species of blue marlin or sailfish. White and striped marlin are separated by about the same level of genetic divergence as Atlantic and Indo-Pacific populations of blue marlin and sailfish, but preliminary analysis of the mitochondrial DNA control region suggests that, unlike Atlantic and Indo-Pacific populations of blue marlin and sailfish, white marlin and striped marlin represent independent evolutionary units. If white and striped marlin are valid species, they are of very recent origin. Significant intraspecific genetic heterogeneity was found among collections of striped marlin and sailfish within the Indo-Pacific; both species exhibited a clear spatial partitioning of genetic variation among geographically distant collection locations. There was no genetic evidence for within-ocean population structuring for other istiophorids examined. Inferences of billfish stock structure derived from studies of molecular markers complement those obtained using other methods of analysis, and together these studies demonstrate substantial differences in the level of population structuring among istiophorid billfishes, information critical for effective management of these highly migratory species.
Length, weight and otoliths of Atlantic and Pacific blue marlin (n = 436) were sampled between 1979 and 1983 in the Atlantic and Pacific Oceans from recreational landings. Otolith weight was highly variable within size classes, but the range of otolith weights was the same for males and females from both oceans. Although age estimates for Atlantic and Pacific blue marlin were not validated, males and females were estimated to be equally long-lived, reaching estimated ages of >20 yr, yet females achieve much larger somatic sizes than males. Length was not useful in predicting estimated age. The sex-related size differences in Atlantic and Pacific blue marlin can be attributed to differential growth between sexes. Although males can grow to a maximum size of ∼ 160 kg and females grow to at least 540 kg, longevity was similar for both sexes. The sex-related size differences in blue marlin are related to differential growth between sexes and are not due to differential mortality. Otolith weight was linearly related to estimated age and r2 ranged from 0.70 to 0.91 for Atlantic and Pacific blue marlin. This relationship demonstrates potential for estimating the age structure of the landings, if validation of the aging method can be achieved.
Some aspects of the reproductive biology of white marlin from the western central Atlantic were analyzed. A total of 2488 fish were sampled by scientific observers on board Venezuelan pelagic longline vessels and from port sampling during the period of January 2002 to December 2006. Reproductive activity was assessed using histology, maximum whole oocyte size; maturity related size frequency distribution of whole oocytes, and a gonadosomatic index. Sex ratio, defined as the proportion of females to males, increased steadily in size beyond 160 cm LJFL and reached 100% at sizes >200 cm. White marlin females with relatively high gonad index values associated with advanced stages of maturity spawned from April to June in the areas NE of Dominican Republic, and N-NE of the Puerto Rico Trench. Estimated sexual maturity (L50) for females was 160.46 cm, and the smallest mature female was 151 cm. Batch fecundity for female fish between 152 and 172 cm LJFL was 190,400–596,200 hydrated oocytes per female. The average interval between spawns was 1.5 days.
The black marlin (Istiompax indica) is one of the largest bony fishes in the world with females capable of reaching a mass of over 700 kg. This highly migratory predator occurs in the tropical regions of the Pacific and Indian Oceans, and is the target of regional recreational and commercial fisheries. Through the sampling of ichthyoplankton and ovaries we provide evidence that the relatively high seasonal abundance of black marlin off the Great Barrier Reef is, in fact, a spawning aggregation. Furthermore, through the tracking of individual black marlin via satellite popup tags, we document the dispersal of adult black marlin away from the spawning aggregation, thereby identifying the catchment area for this spawning stock. Although tag shedding is an issue when studying billfish, we tentatively identify the catchment area for this stock of black marlin to extend throughout the Coral Sea, including the waters of Papua New Guinea, the Solomon Islands, Micronesia, New Caledonia, Kiribati, Vanuatu, Fiji, Tuvalu and Nauru.
Batch fecundity and relative fecundity of swordfish (Xiphias gladius) in the southwestern Indian Ocean were estimated from seven gravid swordfish females (size range 127-225 cm lower jaw-to-fork length, LJFL) with unovulated, hydrated oocytes collected onboard Reunion-based (France) longline swordfish fishing vessels between December 1999 to January 2001. To investigate the spawning pattern of swordfish, we used data collected through a combination of two at sea sampling regimes. A total of 17 007 geo-located size data of swordfish were recorded during 8 years (1993-2001) and a total of 1727 (size range 75-289 cm, LJFL) swordfish gonads (1107 females and 620 males) were sampled from May 1998 to January 2001. The estimated batch fecundity ranged from 995 000 hydrated oocytes for the smallest ripe female to 4.3 millions for the largest female sampled measuring respectively 127 to 225 cm in curved length (LJFL). The relative fecundity ranged from 25 to 72 hydrated oocytes per gram of body weight. We found that batch fecundity was positively correlated with fish length and that the older/larger females have earlier and longer spawning seasons than younger/smaller females. These findings suggested that older/larger females which are seasonally migrating in this spawning ground seem to play a major role in reproductive success of the species in producing significantly more offspring than younger females during an extended spawning season. Examination of the length-frequency date from the fishery indicated that the young fish are resident around Reunion and around the seamounts off Reunion Island. Our results highlight the important role of the older/larger females in the reproductive capacity of southwestern Indian Ocean stock. We discuss the potential implications of fishing the older/larger females for this stock in terms of reproduction and recruitment.
The reproductive biology of blue marlin (Makaira nigricans) was assessed from 1001 fish (ranging from 121 to 275 cm in eye-to-fork length; EFL) caught by Taiwanese offshore longliners in the western Pacific Ocean from September 2000 to December 2001 and from 843 gonad samples from these fish, The overall sex ratio of the catch was approximately 1:1 during the sampling period, but blue marlin are sexually dimorphic; females are larger than males. Reproductive activity (assessed by histology), a gonadosomatic index, and the distribution of oocyte diameters, indicated that spawning occurred predominantly from May to September. The estimated sizes-at-maturity (EFL50) were 179.76 ±1.01 cm (mean ±standard error) for females and 130 ±1 cm EFL for males. Blue marlin are multiple spawners and oocytes develop asynchronously. The proportion of mature females with ovaries containing postovulatory follicles (0.41) and hydrated oocytes (0.34) indicated that the blue marlin spawned once every 2-3 days on average. Batch fecundity (BF) for 26 females with the most advanced oocytes (≥1000 μm), but without postovulatory follicles, ranged from 2.11 to 13.50 million eggs (6.94 ±0.54 million eggs). The relationships between batch fecundity (BF, in millions of eggs) and EFL and round weight (RW, kg) were BF = 3.29 × 10-12 EFL5.31 (r2=0.70) and BF = 1.59 × 10-3 RW 1.73 (r2=0.67), respectively. The parameters estimated in this study are key information for stock assessments of blue marlin in the western Pacific Ocean and will contribute to the conservation and sustainable yield of this species.
Spawning of yellowfin tuna, Thunnus albacares, around Clipperton Atoll, in the eastern Pacific Ocean, occurred between 2230 and 0330 h on the basis of the presence of migratory-nucleus and hydrated-stage oocytes and new postovulatory follicles in ovaries of females sampled at different times of the day. Histological criteria were developed to estimate the ages of the postovulatory follicles and used to estimate spawning frequency. Histological examinations of testicular tissues provided criteria on the structural characteristics of the sperm duct useful for estimation of spawning frequency. The mean interval between spawnings was 1.14 days for females and 1.22 days for males. The average batch fecundity was 1.57 million oocytes, or 68 oocytes per gram of body weight. The average daily cost of spawning, excluding behavioral activities, is estimated to be 0.97% and 0.28% of the body weight per day for females and males, respectively.
Distributions of diatom assemblages in and around a persistent upwelling area off northeastern Taiwan were investigated during summer, 1994. Two water types and two diatom assemblages were defined by principal component analyses. The two water types represent the nutrient-depleted surface water and the nutrient-laden upwelled subsurface Kuroshio Water. During the period of this study, the latter water type did not outcrop but stayed in the subsurface layer, probably resulting from the outflowing of the warm nutrient-poor Taiwan Strait Water. Diatom assemblages indicated the interplay between the two water types. One assemblage, composed of cosmopolitan species in low density, represented the Background Assemblage, which was widely distributed in most of the surface water and also in the underlying upwelling water. The other assemblage, which was observed in the surface water in contact with the underlying upwelling water over the shelf break, represented an Enhanced Assemblage. Some species existing in the Background Assemblage were likely to be enhanced when they were brought into contact with the nutrient-rich upwelled subsurface Kuroshio Water and, subsequently, formed a new assemblage. When the water parcel containing the new assemblage flowed away from the nutrient-rich water, this Enhanced Assemblage sometimes switched back into the Background Assemblage a few days later.
Sagittae, anal and dorsal fin spines were collected from black marlin, Makaira indica, in eastern Australian waters to evaluate and interpret age information from these structures. Presumed annual marks in sections of the third anal and third dorsal spines from 69 fish ranging in size from 8.5 to 451 kg were examined. Sectioned otoliths were also examined for presumed daily microincrements using both light and scanning electron microscopy. Spine radius and sagitta weight each varied predictably with the length and weight of the fish. The time period between consecutive growth marks was checked against the growth of these hardparts recovered from a tetracycline-injected and recaptured fish at large for 6 months. The increase in the size of fin spines from the recaptured fish was consistent with calculated growth curves derived from counts of presumed annual zones. Lengths and weights at age, based on these bands, are presented for black marlin. External ridges were observed on the edge and ventral surface of the rostrum of the sagitta, but they were inconsistently clear to be reliably counted. Sagittae displayed an internal record of microincremental growth on a scale that suggested a daily periodicity. Confident and consistent enumeration of these increments was restricted to fish <40 kg (~1600 mm lower jaw–fork length). A daily cycle of increment deposition was not confirmed by a count from the recaptured fish that fell short of the 198 days at large. The discrepancy was most likely associated with the difficulties of enumerating microincrements. The ages assigned to black marlin in this study support ages suggested from analysis of length–frequency data on juvenile fish. They underline the restraints previously recognized in decoding sagittae of blue marlin and confirm the utility of fin spines in providing age information for istiophorids.
An important component of many studies of fish reproductive biology is the assessment of the stage of gonad development of individual fish. The methods in use vary from highly detailed to cursory, but there are few reviews of their reliability or usefulness. This review examines histology, measurements of oocyte size, staging based on the appearance of whole oocytes, staging based on the external appearance of the ovary, and gonad indices. Histology is the most accurate technique, but it is time- consuming and expensive. Staging based on the appearance of whole oocytes can be a useful alternative but may be inaccurate with oocytes in transitional stages of development. Staging based on the external appearance of the ovary is the simplest and most rapid method, but it may be subjective and its accuracy is uncertain. Oocyte size may be used as a predictor of developmental stage if the size ranges of the various stages are known, but the sizes of different oocyte stages may overlap, which complicates this approach. Oocyte size may be used on its own to measure development but gives little information on the physiological status of the ovaries. Gonad indices (gonad size relative to body size) provide a useful insight into changes in ovary size and complement results obtained using staging methods. However, gonad indices, like oocyte size, may be biased when samples of fish of different body sizes are compared.
Ovaries of bluefin tuna, Thunnus thynnus, were collected from the Gulf of Mexico, Florida Straits, Middle Atlantic Bight of the western North Atlantic, and off the northeast coast of the United States. There was relatively little development towards maturity in age 1 through age 7 fish from the Middle Atlantic Bight as evidenced by low gonosomatic index values and histological examination of ovaries. Well-developed ovaries were present in giant bluefin tuna from the Gulfof Mexico and Florida Straits, with heaviest spawningoccurring in May. For bluefin tuna measuring 205-269 cm fork length and 156-324 kg round weight. the average number of eggs measuring 0.33 mm in diameter and larger was estimated at 60.3 million, and the average number of eggs measuring 0.47 mm in diameter and larger was estimated at 34.2 million. Atlantic bluefin tuna, Thunnus thynnus, are seasonally distributed over most of the North Atlantic. They are found from Newfoundland to Brazil and from Norway to the Canary Islands (Gibbs and Collette 1967). In the western Atlantic, a sport fishery for bluefin tuna exists offthe east coast of the United States from Maine through North Carolina and along the western Bahamasand the eastern coast of Canada. Also, a substantial commercial bluefin tuna fishery exists in the western Atlantic. There is purse seining along the east coast of the United States from Massachusetts to North Carolina and a handline and harpoon fishery off Massachusetts and Maine. A sub stantial Japanese longline fishery is present off the east coast of the United States and in the Gulf of Mexico. In the eastern Atlantic, a sport fishery for bluefin tuna exists around the Canary Islands and a substantial commercial fishery occurs off Europe and North Africa. Purse seining is conducted off the Atlantic coast of Norway and Morocco, the Mediterranean coast of France, the Adriatic coast of Italy and Yugoslavia, in the Tyrrhenian Sea off Italy, and occasionally in the North Seaoff Denmark. An important hook-and line bait fishery occurs in the Bay of Biscay off France and Spain, off Morocco, the Azores, the
The reproductive biology of female bigeye tuna Thunnus obesus was assessed by examining 888 fish (ranging from 84·9 to 174·4 cm fork length, LF ) caught by Taiwanese offshore longliners in the western Pacific Ocean from November 1997 to November 1998 and November to December 1999 and 258 gonad samples from these fish. The overall sex ratio of the catch during the sampling differed significantly from 0·5, but males were predominant in sizes >140 cm LF . Reproductive activity (assessed by histology), a gonado-somatic index, and the size-frequency distributions of whole oocytes indicated that spawning occurred throughout the year and the major spawning season appeared to be from February to September. The estimated sizes at 50% maturity (LF50 ) of females was 102·85 cm (95% c.i.: 90·79-110·21 cm) and the smallest mature female was 99·7 cm LF . They are multiple spawners and oocytes develop asynchronously. The proportion of mature (0·63) and reproductively active (0·70) females with ovaries containing postovulatory follicles indicated that they spawn almost daily. Batch fecundity for 15 females with the most advanced oocytes (>730 µm) ranged from 0·84 to 8·56 million eggs (mean ± s.d. = 3·06 ± 2·09). The relationships between batch fecundity (FB , in millions of eggs) and LF (cm) and round mass (MR , kg) were FB=9·91×10-14LF6·38 (r(2) = 0·84) and FB=8·89×10-4MR2·05 (r(2) = 0·80), respectively. The parameters estimated in this study are key information for stock assessments of T. obesus in the western Pacific Ocean and will contribute to the conservation and sustainable yield of this species.
Pop-up satellite tags were deployed on seven black marlin, Makaira indica,in the north-western Coral Sea, to examine movement, post-capture mortality and habitat preferences. Five of these tags popped up and transmitted positions, and detailed data on diving behaviour, ambient water temperature and daily movement were received via ARGOS transmission from two tags. One tag was later found on a beach, allowing a complete archival data set to be downloaded and geolocation estimates provided by software on-board the pop-up tags and those based on the complete archival tag data sets to be compared. The tags indicated rapid movement away from release sites; three south-easterly displacements (222 km (120 nm), 222 km (120 nm), and 1185.3 km (640 nm) net) appeared to be associated with the East Australian Current, one moved 555.6 km (300 nm) directly east, and the last tag moved offshore and then back towards the coast over a 2-month period for a net displacement of 384 km (207 nm). Based on displacement speeds and diving behaviour, it was concluded that five of the seven fish survived capture and handling for periods ranging from 3 to 64 days. The fate of the other two is unknown. Estimates of longitude made on board the pop-up tag were very similar to the best estimates that could be made using the complete archival data set; however, pop-up tag latitude estimates were significantly more variable than those using the archival data. In the two cases in which pop-up tags were scheduled to stay on the marlin for more than 3 months, the tags detached prematurely, after 39 and 64 days. Temperature and depth data indicated a preference for waters of the mixed layer (20–120 m) and temperatures warmer than 24°C.
The potential annual fecundity of Dover sole becomes fixed before the spawning season when the average diameter of the advanced stock of yolked oocytes exceeds 0.86 mm. About nine batches were spawned over a six-month spawning season (December-May), and spawning ceased when the standing stock of advanced oocytes was exhausted. A 1-kg female released c10 000 eggs per spawning, except for the first and last batches which were smaller than the rest. Near the end of the season, females may spawn more frequently than earlier in the year, increasing the daily production of eggs by the population even though fewer females are reproductively active. Annual reproductive effort of Dover sole was equivalent to c14% of body wet weight per year. Fifty percent of the females had become sexually mature when they reached 332 mm total length. -from Authors
The gonads of 498 istiophorid fishes, mostly from the western North Atlantic Ocean, were examined histologically. Female sailfish (Istiophorus platypterus), white marlin (Tetrapturus albidus), and blue marlin (Makaira nigricans) may spawn up to four times during their sexually active reproductive seasons. Males of all of these species can spawn throughout the year. Histologically, the only difference among the above species and other istiophorids examined—black marlin (M. indica), striped marlin (T. audax), and longbill spearfish (T. pfluegeri)—was in the size of the ova, which is smallest in the sailfish and largest in the blue marlin. The number of previous mature generations of ova can be ascertained by the different stages of involution of atretic ovarian follicles. Indications of previous spawnings are the presence of ruptured ovarian follicles and resorbing ovulated ova. Based upon our histological studies, and on macroscopic field analyses and occurrence of larval stages reported in the literature, the following spawning seasons and weights at first maturity of the female are as follows: sailfish (April through October; 13-18 kg), white marlin (March through June; 20 kg), blue marlin (July through October; 120 kg), and longbill spearfish (November to May; 17-19 kg).
This study presents the first histology-based assessment of the reproductive dynamics of south-west Pacific striped marlin Kajikia audax. Maturity and reproductive status were assessed from histological sections of ovaries (n = 234) and testes (n = 243) of fish caught in commercial longline and recreational fisheries between 2006 and 2009. Spawning peaked in the Coral Sea during November and December at sea surface temperatures between 24·8 and 28·3° C. Lower jaw fork length (L(LJF) ) at 50% maturity (L(LJF50) ), a key variable for stock assessment, was estimated to be 2100 ± 102 mm (mean + s.e.) for females and 1668 ± 18 mm for males. Unlike large pelagic tunas Thunnus spp., the proportion of females increased with length and spawning fish formed multiple large-scale aggregations within a broad latitudinal band. This study provides a starting point for biological parameters needed for stock assessment and conservation of K. audax and introduces the multiple aggregation spawning concept as a reproductive mechanism to explain genetic heterogeneity observed in some highly migratory species.
A tuna longline survey of billfish was carried out in the equatorial western Indian Ocean during 1964-67, in which 265 specimens of five species (Istiophorus platypterus, Tetrapterus audax, T. angustirostris, Makaira indica and M. nigricans) were caught. The comparative morphometric evidence obtained is used in confirming the conspecificity of these istiophorids with Pacific Ocean forms. However, the insufficient depth and detail of the data allow only preliminary observations on populational differences. The distribution and seasonal abundance of the species are discussed in terms of percentage catch rate. The weight-length relations of the species are examined. The size composition of the catches is shown by the analysis of length and weight frequencies, and tentative estimates of the year classes present in the catches of T. audax are given. An examination of the sex ratio of the species is made. Gonad maturation indices for males and females are established, based on both macroscopic and histological evidence. The indices are used to study breeding in the species caught in the survey area. Relative gonad weight is also considered as a means of assessing the stage of gonad development. The results of the gonad examination indicate that little spawning occurs in the area, and it is suggested that seasonal abundance of certain species is related to feeding. Preliminary observations on the size at first maturity are made, and the fecundity of female I. platypterus, T. audax, and T. angustirostris is examined.
Swordfish Xiphias gladius are multiple spawners with asynchronous oocyte development, which results in an indeterminate fecundity pattern. The dynamics of oocyte maturation is typified by seven bootstrapped oocyte distributions that represent the maturation process, and aids in the identification of ovaries with hydrated oocytes that have not spawned. Because of the difficulties in obtaining adequate samples for fecundity estimates in swordfish, the classification of the maturation process becomes very useful when methods for identifying ovulated or post-spawning females, such as histology, are restricted or unavailable.
Gonad examinations were made on billfish caught during a tuna longline survey in the equatorial western Indian Ocean. The species considered were Istiophorus platypterus, Tetrapterus audax, Tetrapterus angustirostris, Makaira nigricans, and Makaira irtdica. The morphology of the urino-genital system is described for all species. It was found to be similar in all except T. angustirostris in which the gonads are Y-shaped rather than bilaterally symmetrical. The histology of the gonads is described, and the changes within the testis and ovary are traced throughout the spermatogenic and oogenic cycles. These are shown to be similar in all species examined.
The ADCP on an advanced towed fish with controllable main and tail wings, called DRAKE measured a detailed sectional structure of the Kuroshio flowing to the NE along the East China Sea shelf slope west of Okinawa. At the observation period, a countercurrent directed to the SW formed in near-bottom water on the shelf slope. The horizontal flow perpendicular to the stream axis of the Kuroshio constructed a convergence zone around the boundary between the Kuroshio and the countercurrent. An intensive upwelling with the maximum velocity of 2.8 cm s–1 was found to distribute on the shelf slope around the convergence zone. A dynamic cause of this intensive upwelling is discussed carefully.
The seasonal occurrence of black marlin, Istiompax indica, around Yonaguni Island, southwestern Japan, was analyzed. Black marlin was abundant from April to July, and the majority
of landed fish were female (97%). None of these females was reproductively mature. The condition factor of females increased
from April to July. Of 56 black marlin examined, 21 individuals contained 12 prey species; the most important prey species
was skipjack tuna, Katsuwonus pelamis, and one neonatal silky shark, Carcharhinus falciformis, was also found. These results indicate that the occurrence of black marlin off Yonaguni Island was associated with feeding
and not with spawning.