Peak Spawning Period Research Articles

Biochemical modulation in male specimens of Channa punctatus (Bloch) under different habitats and seasons

Studies were undertaken to identify the changes in biochemical parameters including water content, lipid and protein in tissues of liver, testis and muscle of Channa punctatus under natural and captive conditions along with physico-chemical parameters for the period January to December, 2008. Result shows that values of lipid in captive condition in muscle, liver and testis ranged from 1 to 4, 1.98 to 5.57 and 1.25 to 4.40%, respectively while in natural habitat, values ranged from 1 to 5, 1.94 to 5.16 and 1.21 to 5.13%, respectively. Water content in captive condition was observed to be in the range of 74.72 to 80.73, 73.97 to 76.92 and 75.56 to 77.97%, respectively and in natural habitat it varied from 72.65 to 80.70, 71.61 to 76.89 and 72.41 to 77.94%, respectively. An increase in water content was observed towards maturation in testis. Protein content in muscle under captive and natural habitat was in the range of 15.97 to 19.37 and 16.70 to 19.40%, respectively. All parameters showed a significant variation (p<0.01) and significant correlation (p<0.01) among the muscles, liver and testis in specimens under captive as well as natural habitat. Peak spawning period in nature was observed during May to August in captive condition indicating that gonadal development and its cyclicity is influenced by the physico-chemical parameters, food and feeding conditions, environmental factors and other stresses. This study shows that the variation in biochemical changes are influenced by variation in seasons due to physico-chemical parameters of the habitat and perhaps the niche. Key words : Habitat, Channa punctatus, testis, biochemical change, physico-chemical parameter.

Orientation behavior in fish larvae: A missing piece to Hjort's critical period hypothesis

Larval reef fish possess considerable swimming and sensory abilities, which could enable navigation towards settlement habitat from the open ocean. Due to their small size and relatively low survival, tagging individual larvae is not a viable option, but numerical modeling studies have proven useful for understanding the role of orientation throughout ontogeny. Here we combined the theoretical framework of the biased correlated random walk model with a very high resolution three-dimensional coupled biophysical model to investigate the role of orientation behavior in fish larvae. Virtual larvae of the bicolor damselfish (Stegastes partitus) were released daily during their peak spawning period from two locations in the Florida Keys Reef Tract, a region of complex eddy fields bounded by the strong Florida Current. The larvae began orientation behavior either before or during flexion, and only larvae that were within a given maximum detection distance from the reef were allowed to orient. They were subjected to ontogenetic vertical migration, increased their swimming speed during ontogeny, and settled on reefs within a flexible window of 24 to 32 days of pelagic duration. Early orientation, as well as a large maximum detection distance, increased settlement, implying that the early use of large-scale cues increases survival. Orientation behavior also increased the number of larvae that settled near their home reef, providing evidence that orientation is a mechanism driving self-recruitment. This study demonstrates that despite the low swimming abilities of the earliest larval stages, orientation during this “critical period” would have remarkable demographic consequences.

Spawning frequency in the eastern Australian river garfish, Hyporhamphus regularis ardelio (Hemiramphidae)

Patterns in the timing of spawning of the eastern Australian river garfish Hyporhamphus regularis ardelio were investigated by sampling fish throughout 24 h during their peak spawning period. Patterns in gonadosomatic indices, the presence of hydrated oocytes and post-ovulatory follicles in histological ovary sections, together with previously demonstrated polymodal distributions of oocytes undergoing final maturation, indicated that river garfish were capable of spawning at any time during the 24 h period. Spawning had either recently occurred or was imminent in all females sampled suggesting that river garfish could spawn multiple times in a given season. Although energy resources are used in the production of low numbers (<2500) of large (~2.5 mm diameter) eggs per batch, total fecundity during the reproductive season may be elevated through multiple spawning events. Implications of these findings on current fishery management regulations are discussed.

Observations on the Composition, Physiological Condition and Fisheries in Erinle Lake, Osun State, Nigeria

The level of fish production in Nigeria is far less than the current fish demand. The study, therefore, on fish composition and fishing activities in Erinle Lake is meant to suggest means of bridging the gap between fish supply and demand in the country. Fish samples were obtained from gill-net, cast net and traps. Ten families of fish comprising 19 species were identified, dominated by Cichlid fish (71%) largely made up of Sarotherodon galilaeus (42%). The sex-ratio of most fish in the lake tends to unity (1:1), suggesting reproductive stability. Condition factors are high for the fish species &ndash; an indication that they thrive well in the habitat. Key water quality parameters such as pH, DO, etc. were found to be favourable for survival and thriving of food for some fish species. Fishermen on the lake used 2.54 cm mesh size net contrary to 7.62 cm mesh size recommended in Sea Fishing Regulations of 1971, which is a supplement of Nigerian Sea Fisheries Decree (ACT) of 1971. As a result the lake had been exploited of fish resulting in a low and small size catches. It is recommended that the fisheries be closed during the peak spawning period to allow possible recovery. Also, strict monitoring control and surveillance of the lake is highly recommended, most especially in the use of unapproved fishing nets and methods.

Elemental fingerprints of southern calamary (Sepioteuthis australis) reveal local recruitment sources and allow assessment of the importance of closed areas

Movement of individuals over a range of temporal and spatial scales is a critical process in determining the structure and size of populations. For most marine species, a substantial amount of movement that is responsible for connecting subpopulations occurs when individuals are too small and numerous to be tagged using conventional methods. Using the elemental fingerprints of the statoliths of the squid Sepioteuthis australis and a robust machine learning classification technique, this study determined that newly hatched squid had elemental signatures that exhibited sufficient spatial variation to act as natural tags for natal origin and that elemental signatures can be used to allocate adult squid back to their natal site. Between 55% and 84% of the adult squid caught throughout the east and southeast of Tasmania, Australia, were classified back to an area that is closed to commercial fishing over much of the peak spawning period, and this was the only location with substantive evidence of natal recruitment. Although many studies have demonstrated the potential of this approach to discern connectivity between population units, few studies have successfully done so by then examining the trace element profiles of adults in addition to those of hatchlings as we have demonstrated with S. australis.

Aspects of the biology and fisheries of an economically important sparid Dentex macrophthalmus (Bloch 1791) in the Namibe province, Angola

The sparid Dentex macrophthalmus is a widespread, important fishery species along most of the West African coast from southern Namibia to the Mediterranean. In southern Angola it is an important artisanal species targeted predominantly by handline fishers. A biological and fisheries study was conducted on this species in southern Angola between June 2008 and July 2009. It was the dominant species in the artisanal fishery, accounting for 99% of the sparids captured and 67% (by mass) of the total catch. The life history of D. macrophthalmus was characterised by slow growth (females: Lt = 309[1 − e−0.06(t + 5.43)], males: Lt = 248[1 − e−0.16(t − 1.77)]), advanced age at maturity (females: 7.4 years, males: 6.0 years) and high longevity (females: 36 years, males: 38 years). The sex ratio was 1:1 male:female. The length- and age-frequency distributions and macroscopic observations suggested that the species is a late gonochorist. Males and females reached 50% maturity at 151 and 166 mm fork length respectively. Although individuals with ripe gonads were found during most of the year, the peak spawning period appeared to be in December and January. Despite a life history that renders D. macrophthal-mus vulnerable to overexploitation, only 38% of artisanal fishers noticed a decline in the catches of this species. Potential reasons for this include: technology creep; limited pressure on the juvenile portion of the stock as a result of late recruitment (above the length of 50% maturity); the ‘basin affect’, whereby depleted areas are reseeded by areas (e.g. deeper water) inaccessible to the linefishery; and a deep-water reserve of large individuals. It is recommended that precautionary management strategies be implemented until the age estimates are revised by the countries in which this species forms significant fisheries.

Assessment of Artisanal Fishery Impacts on West African Croaker Pseudotolithus elongatus in the Cross River Estuary, Nigeria, Using Length‐Based Models

AbstractIn Nigeria and other countries of the Gulf of Guinea, the genus Pseudotolithus is predominant in the catches of both artisanal (small‐scale, commercial, or subsistence) and industrial fisheries. Data on this fishery are readily available, enabling resource managers to track stock dynamics. The study objective was to ascertain the status of an exploited stock of West African croaker (also known as the giant captainfish) P. elongatus using von Bertalanffy growth models. Length distribution data were collected monthly from landings of artisanal fisherfolk at the Cross River estuary, Nigeria, between March and August 2008. A fisheries stock assessment tool developed by the United Nations' Food and Agriculture Organization was used to calculate estimates of the von Bertalanffy growth function based on these data. The asymptotic length was 61.5 cm, the curvature parameter was 0.40/year, the oscillation constant was 0.3, the winter point was 0.40, and the goodness‐of‐fit index was 0.515. The instantaneous rates of total, natural, and fishing mortality were 3.71, 0.78, and 2.93 per year, respectively. The resultant exploitation ratio was 0.79, indicating that the fishery is overexploited. Management regulation, particularly seasonal closures coinciding with the period of peak spawning, should be implemented to avoid the eventual collapse of this fishery. Particular attention should be given to the conservation of the mangrove habitat that sustains the productivity of this system. Previous recommendations for the management or conservation of this stock have neglected the depletion of the mangrove ecosystem supporting this species. It is imperative to include habitat conservation as one of the regulatory measures for this stock.

Reproductive cycle in female Waigieu seaperch ( Psammoperca waigiensis) reared under different salinity levels and the effects of dopamine antagonist on steroid hormone levels

Waigieu seaperch ( Psammoperca waigiensis) is a tropical marine finfish species, which may inhabit wide range of salinity during the entire life cycle. Regardless of the wide salinity tolerance, little is known about how salinity may influence the reproductive endocrinology of this important tropical species. In the present study, we investigated the seasonal variations in steroid hormone levels, oocyte maturation (OM) and ovulation in fish reared under different salinity levels. In addition, we investigated the effects a dopamine antagonist (domperidone: DOM) during the peak spawning period. Mature brood fish at 3 years old were cultured in four different salinities of 5, 10, 20 and 30‰ (part per thousand, ppt) from March to December 2007. Blood samples were collected monthly and key steroid hormones (testosterone (T), 11-ketotestosterone (11-KT), 17β-estradiol (E2) and progesterone (P)) levels in female fish plasma were analyzed using enzyme immunoassay method. OM, ovulation and several reproductive performance indexes were evaluated twice per month during the breeding season. Plasma hormone analysis showed significant differences in fish groups cultured in different salinities during the seasonal cycle. The gonadosomatic index (GSI) gradually increased from March, peaking in July at 10 and 30‰, and then decreased thereafter until December. Plasma steroid levels (E2, T, 11-KT and P) were observed in variable concentrations during the spawning period and showed monthly fluctuations that were apparently salinity dependent during on and off-seasons. Interestingly, 11-KT levels were found in small amounts in female fish and peaked in September at salinities of 10 and 20‰. DOM exposure produced significant differences in steroid hormone levels that were apparently dependent on dose, exposure time and the individual hormone. The present study indicated that holding Waigieu seaperch brood fish in different salinities during the breeding season produced significant effects on gonadal development and spawning incidences. The first spawning was observed on 8th of April for all groups after 100% water exchange at night. No significant differences of absolute and relative fecundity were observed between groups. Fish held at 10 ppt salinity during the breeding season spawned naturally, but maturity and spawning rates were reduced and no hatching was observed at the same salinity. Fish holding at 5‰ resulted to 100% mortality in brood fish. This indicated that salinity limits for oocyte maturation or ovulation and subsequent reduction in spawning, but embryonic development requires a higher salinity. The effect of salinity on embryo development and hatching rate suggests that in breeding season the brood fish spawned in areas where the salinity and other parameters are favorable.

Reproductive biology of two co‐occurring mugilids, Liza argentea and Myxus elongatus, in south‐eastern Australia

The reproductive biology of Liza argentea and Myxus elongatus occurring in two estuaries (Lake Macquarie and St Georges Basin) was found to differ. Gonado‐somatic index values and macroscopic staging of gonads identified the peak spawning period of L. argentea occurred between March and November in Lake Macquarie and January and April in St Georges Basin. In contrast, peak spawning of M. elongatus was concentrated between January and March in both estuaries. Spawning of L. argentea probably occurred in the lower reaches of estuaries as well as in nearshore coastal waters, whereas evidence indicated M. elongatus spawned only in ocean waters. The mean fork length at maturity (LF50) was greater for females than males in both species, and it also occurred at a larger mean LF in M. elongatus (males = 230 mm and females = 255 mm) than L. argentea (males = 180 mm and females = 207 mm). Estimates of total potential fecundity were also greater for M. elongatus (425 484–1 157 029) compared to L. argentea (159 933–548 954). Both species had determinate fecundity and displayed a group synchronous pattern of oocyte development, with two distinct size classes of oocytes present in mature ovaries. Liza argentea probably release the larger class of oocytes in one spawning event, but this could not be established for M. elongatus.

Experimental harvesting of Mytilus galloprovincialis: Can an alien mussel support a small-scale fishery?

In an effort to stimulate new fisheries and address historic imbalances in access to fishing rights, there has been a recent focus on the development of small-scale fisheries in South Africa. To assess the biological viability of a fishery for the alien mussel Mytilus galloprovincialis, an experimental fishery operated by two impoverished coastal communities was initiated. Harvesting took place on a rotational basis at three sites, nested within four locations. At each of these 12 sites, 5 treatments were undertaken to span a spectrum of harvesting intensities ( F = 0, 0.3, 0.6, 0.9 and a once-off total removal). A dynamic biomass-based fisheries model was developed to predict changes in exploited populations over time. Monthly maximum sustainable yield (MSY) estimates peaked at 1560 kg per 100 m of shore in March–April and September–October, but dropped by two orders of magnitude to 15 kg per 100 m of shore during the remainder of the year. The two peaks in MSY corresponded to the peak spawning periods of M. galloprovincialis along the South African west coast. Consequently, harvesting will only be viable if focused within two seasons spanning the peaks in MSY. Under these conditions, a range of harvesting intensities between F = 0.1 and 0.3 would permit long-term biological sustainability of a fishery. If implemented, this would represent the first instance of a marine invasive species being utilised to achieve socio-economic goals in South Africa.

Feeding and management practices affect quality improvement in wild-caught edible crab ( Cancer pagurus)

About 80% of the Norwegian landings of edible crab ( Cancer pagurus) are females. Meat content is often poor also for hard-shelled crabs. Feeding has proved to increase the quality, however the feeding process is costly and should be as short as possible still providing the required product quality. The crab fishery season coincides with the females' reproduction period which affects the quality and body composition. Egg-carrying crabs are of no commercial value. The development of the gonads is therefore of great importance for a feeding management of crabs. A study was conducted to test if meat yield and quality increased over time in the peak season (September). Groups of females were fed a diet of saithe ( Pollachius virens) fillet for 10, 24 and 33 days. An unfed reference group was killed, boiled and quality assessed at the start of the experimental period and compared with fed groups assessed in the same way at the termination of the feeding period. The improvement of quality traits was a continuous process, with a linear increase in total subjective impression, hepatopancreas lightness and gonadosomatic index (GSI) correlated with time of feeding or cumulative feed intake. Two different methods of inactivating the claws, banding and nicking, were compared during the feeding period. The treatments gave no difference in feed intake or external quality like lost legs and tips. Gonad and hepatopancreas were examined during several short experimental periods (3–4 weeks) expanding from just before until after the peak fishery season. In August, September and October/November GSI increased but no increase was seen in hepatosomatic index (HSI). Crabs with GSI between 15 and 18 (%) seemed to be close to spawning. In October/November 40% of the crabs spawned, in December 6.7%. No development in GSI was present in December but HSI increased for fed crabs. After the peak spawning period in October/November most of the remaining crabs do not start the onset of gonad development before the following year. Fed crab at this time of the year will be of a different body composition, with more hepatopancreas and less roe. Several factors may influence the economy in a feeding program. However, the short feeding period makes it possible to feed several batches in a year. In a growing market and with increasing knowledge in quality grading technology of live crabs, feeding of edible crab may be worthwhile in the future.

Spawning Habitat Associations and Selection by Fishes in a Flow‐Regulated Prairie River

AbstractWe used histological features to identify the spawning chronologies of river‐dwelling populations of slenderhead darter Percina phoxocephala, suckermouth minnow Phenacobius mirabilis, stonecat Noturus flavus, and red shiner Cyprinella lutrensis and to relate their reproductive status to microhabitat associations. We identified spawning and nonspawning differences in habitat associations resulting from 1 year of field data via logistic regression modeling and identified shifts in microhabitat selection via frequency‐of‐use and availability histograms. Each species demonstrated different habitat associations between spawning and nonspawning periods. The peak spawning period for slenderhead darters was April to May in high‐velocity microhabitats containing cobble. Individuals were associated with similar microhabitats during the postspawn summer and began migrating to deeper habitats in the fall. Most suckermouth minnow spawned from late March through early May in shallow microhabitats. The probability of the presence of these fish in shallow habitats declined postspawn, as fish apparently shifted to deeper habitats. Stonecats conducted prespawn activities in nearshore microhabitats containing large substrates but probably moved to deeper habitats during summer to spawn. Microhabitats with shallow depths containing cobble were associated with the presence of spawning red shiners during the summer. Prespawn fish selected low‐velocity microhabitats during the spring, whereas postspawn fish selected habitats similar to the spawning habitat but added a shallow depth component. Hydraulic variables had the most influence on microhabitat models for all of these species, emphasizing the importance of flow in habitat selection by river‐dwelling fishes. Histological analyses allowed us to more precisely document the time periods when habitat use is critical to species success. Without evidence demonstrating the functional mechanisms behind habitat associations, protective flows implemented for habitat protection are unlikely to be effective.

Reproductive biology of &lt;i&gt;Citharinus citharus&lt;/i&gt; in Onah Lake, Asaba, Delta State of Nigeria

The breeding cycle of Citharinus citharus was investigated from 432 specimens collected from Onah Lake, in Asaba, Delta State of Nigeria. This species formed the mainstay of the fishery resources of the lake. The reproductive biology of this species was analyzed using standard techniques. The sexes were dominated by the males in a ratio of 1:1.4 suggesting that the males probably live longer than the females. Six matured males and females each were encountered. Only four females out of this number was in the spawning stage while the remaining two females were in the spent condition. All six males encountered had their gonads in the spent condition. The size at maturity was not similar in both sexes. The females mature earlier at a standard length of 17.5cm than the males (19.0cm). The youngest spawning female had a standard length of 30.0cm and a body weight of 1310.0g while the youngest spawning male had a standard length of 32.2cm and a body weight of 899.6g. The survival of this species was guaranteed because it had a high fecundity ranging between 14,320 and 16,009. The relationships between fecundity and standard length, fecundity and corrected body weight were described. The gonado-somatic index (G.S.I) was higher in female than in male specimens throughout the period of study. This species was a total spawner with the spawning stock encountered only during heavy rains (May–July). Only one period of peak spawning was established for this species in May/June while the recruits became ready for spawning after three years. Tropical Freshwater Biology 2003/2004 Vol. 12/13: 119-129

Population structure and reproduction of the African bonytongue Heterotis niloticus in the Sô River‐floodplain system (West Africa): implications for management

Abstract – Population structure of the African bonytongue, Heterotis niloticus, was examined in southern Benin within Lake Hlan and a region of the Sô River floodplain located approximately 60 km downstream from the lake. Both locations support important fisheries in which Heterotis is the principal target species during the flood period. Ripe adults comprised over 40% of the population in Lake Hlan, whereas only 3.5% of individuals captured from river sites were adults. Monthly averages for the gonadosomatic index and percentages of individuals with mature gonads peaked as water levels increased during the wet season then declined during the peak flood period. Oocyte size frequency distributions from ovaries suggested a potential to produce an additional cohort in the event of nesting disruption. During the peak spawning period (May to August), between 37 and 51 active nests per hectare per month were observed in Lake Hlan. The number of larvae per nest ranged from 3953 to 6125. Lake Hlan bonytongues appear to constitute an important source subpopulation that exports new recruits to river/floodplain areas downstream where intense fisheries harvest mostly juveniles and subadults. Consequently, restriction of harvest of adult bonytongues in Lake Hlan may be essential for sustenance of commercial fishing in downstream reaches of the Sô River.

An exceptional haddock year class and unusual environmental conditions on the Scotian Shelf in 1999

The number of settled year-0 haddock on the eastern Scotian Shelf (ESS) in July 1999 was the highest seen since record keeping began in 1970. The average length indicated that most had hatched in late February, well before the historical peak spawning period. In the data record above average year-0 abundance leads to good recruitment, suggesting that early spawning incurs survivability advantages. Conditions may have been particularly favourable for haddock recruitment in 1999: (i) because an unusually large number of adults spawned early and (it) because the earliest larval stages encountered an unusually plentiful supply of food, since the spring bloom and Calanus finmarchicus reproduction peaked in late February, 2-4 weeks earlier than usual.

Cohort‐Specific Growth and Mortality of Juvenile American Shad in the Pamunkey River, Virginia

AbstractWe estimated the variation in instantaneous rates of growth (G) and mortality (M) between intraannual cohorts of juvenile American shad Alosa sapidissima in the Pamunkey River, Virginia. The ages of juveniles captured by push net during the juvenile abundance index surveys in 1998 and 1999 were estimated by counting daily rings on the sagittal otoliths. Weight‐at‐age and abundance‐at‐age data were used to generate instantaneous daily rates of growth and mortality for 5‐d cohorts. In 1998, the peak hatch date lagged behind the peak spawning periods that had been inferred from collections of American shad broodstock and occurred after peak spring flow events. In 1999, the correspondence between the hatch date distribution and the peak spawning periods was greater than in 1998. The instantaneous daily growth rate was relatively constant between cohorts, ranging from 0.037 to 0.064 in 1998 and from 0.046 to 0.066 in 1999. The instantaneous daily mortality rate was more variable between cohorts, ranging from 0.047 to 0.084 in 1998 and from 0.044 to 0.093 in 1999. The physiological mortality rate, or M/G, was calculated for all cohorts. Most cohorts in 1998 and 1999, including the largest cohorts in both years, had an M/G value close to 1.0, indicating that these cohorts were barely maintaining or losing biomass during the early juvenile stage. The results of this study indicate that in both 1998 and 1999 the year‐class declined in biomass during the early juvenile stage and underwent demographic structuring that affected the composition of the population that ultimately migrated to sea.

Cyclic changes in gonadal maturation and histological observations of threatened freshwater catfish "narikeliru" Mystus montanus (Jerdon, 1849)

Background. Usage of biosciences in increasing fish production needs to have a proper understanding and knowledge of endocrine physiology of fish reproduction. The most suitable method of determining the reproductive cycle in fishes is to observe seasonal development changes in the gonads. The present paper describes morphological changes in gonads of Mystus montanus. Materials and Methods. The individuals of M. montanus were sampled monthly in captive condition throughout one year to determine the changes occurring in gonadal histology and reproductive status. The stages of gonadal maturation and the seasonal changes in the proportion of oocyte development within the ovaries were noticed and maturity stages were assessed by microscopic and also macroscopic observations. Results. The peak spawning period of Mystus montanus was noticed during November–January in male and October–December in female. The size at first maturity was 10–11 cm (8–12 g) in male and 13–14 cm (14–16 g) in female, respectively. M. montanus spawned only once in a year with the onset of north-east monsoon. The gonadosomatic index (GSI) ranged from 2.8 to 8.5 in males and from 4 to 16 in females and their condition factor (CF) was 4–6 and 6–9, respectively. Conclusion. The sequence of gonadal maturation in M. montanus is morphologically and histologically divided into five stages viz. immature, maturing, mature, matured, and spent or rest.

Studies on some reproductive aspects of &lt;i&gt;Pseudotolithus elongatus&lt;/i&gt; in Cross River estuary, Nigeria

To determine the spawning time of Pseudotolithus elongatus the Cross River estuary, ovarian development, egg diameter, gonadosomatic index (GSI) and hepatosomatic index (HSI) of the female fish were studied from January to December 1998. All ovary developmental stages, including the spawning stage, were present every month, which indicated year-round spawning. High mean values of GSI and egg diameter in December-March and July-September indicated these as the two peak spawning periods for the species. The egg diameter ranged from 1 µm to 590 µm. The highest number of eggs found in an individual fish was 808911. Absolute fecundity of the species in The Cross River estuary can be calculated in relation to standard length using the formula F = 17557SL 0.768 or F = 45532W 0.277 in relation to weight. There were evidences to suggest that the species is an asynchronous partial spawner. The stock might be effectively conserved by the imposition of a closed fishing season during the two peak spawning periods.

Reconstruction of trophic pathways between plankton and the North Iberian sardine (&lt;i&gt;Sardina pilchardus&lt;/i&gt;) using stable isotopes

SUMMARY: Feeding on phyto- and zooplankton by juvenile (< 1 year old) and adult sardines (Sardina pilchardus) was inferred from analyses of natural abundance of stable carbon and nitrogen isotopes in samples from the northwestern Iberian Peninsula (Spain) collected at the beginning of the upwelling season and peak spawning period of sardine. Plankton samples were fractionated through nets of 20, 200, 500, 1000 and 2000 µm mesh-size and the muscle protein of individual sardines was isolated before isotopic determinations. Up to six planktonic components and two sardine feeding types were identified from the modes in the frequency distributions of isotope abundance values. Also, the most probable pathways for carbon and nitrogen flows between compartments were analysed. The resulting food web revealed a relatively large degree of omnivory, both in plankton and sardine components, which confirms that complex trophic interactions could also occur in pelagic upwelling ecosystems. Young sardines had isotope abundance values clustered around a single mode in the frequency distribution, while adult sardines displayed two main modes. These modes are interpreted as representative of two extreme feeding types: one related to the individual capture of zooplankton prey and the other to unselective filter-feeding. Although both types of feeding could include micro- (20-200 µm) and mesozooplankton (200-2000 µm) prey, phytoplankton appears to be ingested mainly by filter-feeding. However, even adult sardines must be mainly zoophagous to achieve the observed isotopic abundance values, taking into account current assumptions on stable isotope enrichment through trophic levels. From the differences in the resulting pathways using either carbon or nitrogen isotopes, we interpreted that sardines acquire most of the protein nitrogen from zooplankton while a substantial fraction of their carbon would derive from phytoplankton. These interpretations agree with the information available for this species on the gut contents and the anatomy of the filtering apparatus.

Coexistence and reproductive isolation of the sympatric echinoids Diadema savignyi Michelin and Diadema setosum (Leske) on Kenyan coral reefs

The sympatric echinoids Diadema savignyi and D. setosum coexist in shallow reef lagoons throughout East Africa. The reproductive strategies of these echinoids were studied to investigate reproductive isolation as a possible mechanism for maintaining the coexistence of these closely related species. The annual reproductive cycle and lunar periodicity were determined by gonad index measurements, histological examination of gametogenesis, and induction of spawning with injections of KCl. The peak reproductive period of D. savignyi coincided with the north-east monsoon period (when light and temperatures are high) as gonad indices were high (>8%) beginning in February and peaked at 9.7% in May. Gonad indices subsequently rapidly decreased (by 26%) in June at the beginning of the cooler south-east monsoon period. However, the presence of sperm and ova in most months of the year indicates continuous gametogenesis with reduced reproductive effort during the cooler months. The annual cycle for D. setosum showed less of a seasonal trend as gonad indices remained above 7% throughout much of the year but tended to be highest when temperatures were lower. This is the first confirmation of continuous reproduction in these two species at the equator. The reproductive patterns of both species remained consistent over 2 years of sampling. Both species exhibited a synchronized lunar spawning periodicity during the 3 months sampled, with D. setosum spawning on lunar days 8–10 and D. savignyi spawning after the full moon (lunar days 17–18). Whereas spawning in D. savignyi was very tightly synchronized, 20% of D. setosum individuals still spawned after the peak spawning period. The coexistence of these closely related species appears to be maintained by temporal reproductive isolation during the lunar spawning period reinforced by seasonal differences in reproductive effort.