The role of mobile consumers in lake nutrient cycles: a brief review

Spawning habitats of anchovy Engraulis encrasicolus and sardinella Sardinella aurita were investigated in the Gulf of Gabès by means of two surveys carried out during the summer of 2005 and 2009. The spatial patterns of early developmental stages were analyzed through Lloyd’s patchiness index, single quotient analyses, and principal component analyses. The results showed that Gulf of Gabès is an important area for anchovy and sardinella spawning. Within this area, anchovy and sardinella showed preference for the warmest waters to spawn. The main spawning areas for anchovy were located in the inner parts of the Gulf, and a secondary spawning area was observed offshore. Contrastingly, the main spawning ground of sardinella was found in the center of the Gulf. In both years, early larvae of anchovy showed a greater degree of aggregation than that of sardinella. The developmental stages of both species showed low spatial overlap, indicating that the spawning habitats of these species are spatially differentiated in the Gulf of Gabès.     

Common dolphinfish,Coryphaena hippurus,spawn at their easternmost Atlantic range adjacent to the Mediterranean Sea

This study provides sound evidence of the spawning activity of common dolphinfish (Coryphaena hippurus) in the Gulf of Cadiz (southwest Spain) during summer months. Along with waters off Côte d'Ivoire, this area adjacent to the Mediterranean Sea is the easternmost Atlantic spawning ground documented for the species. All the fish analysed (5 males and 15 females) were at spawning stage, except the smallest female (61.5 cm in fork length), which might not have yet reached sexual maturation. The oceanographic conditions during the samplings were similar to those occurring during the peak spawning season in the Gulf of Mexico. The study results are consistent with recent genetic studies indicating differentiation between Mediterranean and Atlantic dolphinfish populations, with the Strait of Gibraltar representing a boundary between two distinct reproductive units. The present results may contribute to improve the understanding of the dynamics of dolphinfish population and the management of eastern Atlantic and Mediterranean stocks.     

Role of bay, fjord and seamount on the early life history of Lepidonotothen squamifrons from the Kerguelen Islands

Among the Kerguelen Islands' demersal fish, larvae of Lepidonotothen squamifrons are dominant during the summer over the island shelf and surrounding seamounts. Distribution of larval stages from different scientific surveys confirmed the position of the two known spawning grounds (south of the Kerguelen shelf and at the Kerguelen-Heard bank) and may indicate other spawnings on the Skiff bank and in bays and fjords. Different larval cohorts were observed for the shelf and the coastal zone whereas a unique one was observed for Skiff and Kerguelen-Heard seamounts. Larvae from the spawning ground south of the Kerguelen shelf appear in early summer and show a northwards dispersal to the northeastern nearshore zone, the Baleiniers Gulf, following the scheme of fish migration shown by Harden-Jones. Later on, larvae from the northern inshore spawning zone occur in the same area, which presumably then avoid intraspecific competition with the previous larvae. Larval distribution and migration strategy seemed to be in accordance with the island mass or seamount effect under a regular current. Accepted: 2 December 1999     

Implications of early life stages in the natural history of the Gulf of Mexico sturgeon,Acipenser oxyrinchus de sotoi

Synopsis Gulf of Mexico sturgeon were collected during their natural spring spawning migrations from the Gulf of Mexico into the Suwannee River in Florida. Peak numbers were observed during March and April. During the summer periods the fish remained in the river over very localized bottom areas. In late fall, the fish migrated from the river into the Gulf of Mexico. Laboratory experiments revealed that maximum survival of eggs, embryos and larvae of Gulf of Mexico sturgeon occurred between 15 and 20° C. Although Gulf of Mexico sturgeon appear to have a high tolerance to warm water temperatures, their embryos and larvae exhibit high mortalities at temperatures of 25° C. Based on observations of Gulf of Mexico sturgeon migratory patterns and laboratory experiments we suggest that the timing and unusual migratory behavior of Gulf of Mexico sturgeon are associated with temporal water temperature changes in the river and the Gulf of Mexico. The requirements of a thermal refuge are discussed.     

Interannual differences in growth and hatch date distributions of age-0 year walleye pollock Theragra chalcogramma (Pallas) sampled from the Shumagin Islands region of the Gulf of Alaska, 1985 – 2001

Daily increments of age-0 year walleye pollock Theragra chalcogramma otoliths from 1985 to 2001 were examined for interannual variability in growth and hatch dates. Fish were collected in summer and autumn surveys near the Shumagin Islands in the western Gulf of Alaska (GOA). Hatch date distributions of these fish were compared with hatch dates of larvae from spawning aggregations located in Shelikof Strait and the Shumagin Islands based on their spawning times. The hatch date distributions of age-0 year fish captured from the Shumagin Islands area were similar to those observed for larvae from the Shelikof Strait spawning group. Age-0 year fish whose hatch dates corresponded to the Shumagin Islands spawning, which occurred earlier in January and February, were not found. Sea surface temperature was associated with variability in hatch date distribution and growth.     

Spawn and spat collecting relationship in Mimachlamys gloriosa (Reeve, 1853) and Brachtechlamys vexillum (Reeve, 1853), Pectinids of New Caledonia

Pectinidae spat collecting was carried out during a period of 1.5 years in the southern lagoon of New Caledonia. The majority of spat collected were constituted by Mimachlamys gloriosa and Brachtechlamys vexillum larvae. Settlement showed quantitative variations during the year with a maximum in June-September and a minimum (close to zero) in summer (February). However, although adult stocks maintain a stable density and spawn all the year round, spawning intensity shows a maximum in austral summer, contrarily to the recruitment. Therefore, spawning and spat collecting present no correlation and high values of the gonadic index are not followed by numerous spat collecting. These two tropical species present an opportunistic reproductive model.     

Premetamorphic bonefish (Albula sp.) leptocephali from the Gulf of California with comments on life history

Synopsis Ten early premetamorphic bonefish (Albula sp.) leptocephali were collected in MOCNESS plankton tows in the Guaymas Basin area of the Gulf of California from 27 July–1 August, 1985. Initial captures of five larvae showed that these were found only in surface waters (0–100 m) at temperatures of 15–29° C. A shallow tow made from 200 m to the surface resulted in the capture of five additional larvae that were distributed in the upper 50 m of the water column at temperatures of 21–29° C. Both pre-flexion larvae (<17.0 mm notochord length, NL) and larvae with flexed notochords were captured. The size range was from 12.0 mm NL to 27.0 mm standard length. Drawings of representative larvae are given. This is apparently the first report of premetamorphic bonefish larvae for the Gulf, although metamorphic larvae have been known to be abundant in coastal areas of this region for almost 100 years. Distribution records for metamorphic larvae are reviewed. This, together with observations on seasonal distribution of ripe adults in coastal waters near Guaymas, Sonora, Mexico, has allowed us to speculate on various aspects of life history. Our data suggest that spawning occurs during late spring and summer and that early development takes place offshore. The premetamorphic interval is postulated to last approximately 6–7 months. Larvae then return to coastal areas during the winter and spring to complete metamorphosis.     

Reproductive cycle of female yellow snapper Lutjanus argentiventris (Pisces, Actinopterygii, Lutjanidae) in the SW Gulf of California: gonadic stages, spawning seasonality and length at sexual maturity

The yellow snapper ( Lutjanus argentiventris ) in the SW Gulf of California is being heavily fished and little information exists on status of this exploited population. From August 2001 to May 2003, 440 specimens in the size range of 10.8–59 cm were collected from four fishing areas: Huizache-Caimanero, Mazatlan, Marmol and Santa María la Reforma in the SW Gulf of California. Specimens were collected from fishermen and obtained by sampling with gill nets, hook and line and spear gun fishing. Gonadal stages, spawning season and length-at-first maturity were estimated for female yellow snapper through histological analyses and relative gonadosomatic (GSI) and hepatosomatic (HSI) indexes. Six gonadal stages were identified and gonadal development was asynchronous. Gonads in advanced stage VI of vitellogenesis occurred during summer and winter together with the highest values of the GSI and HSI, indicating that spawning occurred during these seasons. Mean maturity length was determined to be 32.6 cm total length. The yellow snapper population was being adversely affected considering that the species was exploited throughout the year, including the reproductive seasons, and that organisms as small as 10 cm were found in fishermen's catches and at fish markets. Minimum capture sizes of 33 cm and a closed season during the reproductive periods of summer and winter are suggested as measures to reduce the adverse affects of this exploitation.     

Temperature-driven changes in early life-history stages influence the Gulf of Riga spring spawning herring (<Emphasis Type="Italic">Clupea harengus m</Emphasis>.) recruitment abundance

Processes occurring during early life-history stages influence the year-class abundance of marine fish. We found that the abundance of 1-year-old spring spawning herring is statistically significantly determined by the number of post-flexion herring larvae in the Gulf of Riga (Baltic Sea). The abundance of consecutive developmental stages of larvae: yolk-sac, pre-flexion, flexion and post-flexion strongly correlated with each other, indicating that factors which already influence the yolk-sac stage are important in determining the abundance of post-flexion herring larvae. Winter air temperature before spawning determined the timing of maximum abundance of pre-flexion herring larvae, but not their main prey: copepod nauplii, implying that different mechanisms governing major preconditions for the formation of year-class strength. The abundance of post-flexion larvae displayed a potential dome-shaped relationship with sea surface temperature experienced after hatching. We suggest that increased summer temperatures, which exceed the physiological optimum negatively, affect the survival of post-flexion herring larvae. Overall, future climate warming poses an additional risk to larval herring survival and this may lead to a reduction in those herring stock which rely on recruitment from shallow coastal areas.     

Ecology of larval herring in relation to the oceanography of the Gulf of Maine

The dynamics of larval herring in the north-eastern Gulf ofMaine are reviewed with reference to the important physicaland biological oceanographic processes of the region. Particularattention is given to the apparently conflicting hypothesesof larval drift from the tidally well-mixed spawning areas tonursery areas, and larval retention in spawning areas for aperiod of several months. Both processes have been reportedfor the eastern Maine-Grand Manan spawning area of the Gulfof Maine, but the relative importance of each to larval survivalthrough the winter and recruitment to the juvenile stage isnot clear. Both transport and retention are interpreted in lightof oceanographic processes that might impart variability, especiallybetween years, in the proportion of larvae transported awayversus that retained. Results of recent survey cruises in theGulf of Maine show both hypotheses to have merit. It is suggestedthat (i) interannual differences in slope water intrusions intothe Gulf of Maine as they affect the residual circulation, (ii)the lunar periodicity in the intensity of tidal mixing in relationto hatching times, and (iii) the potential for variable spawninglocations relative to the tidal fronts, may affect the distributionsof herring larvae immediately after hatching in the fall, andmay control the proportion of larvae that are advected awaywith the residual currents versus that retained in the vicinityof spawning. It is also suggested that those processes thataffect larval distributions and survival in the fall are importantin determining the overwintering distributions of larvae inthe Gulf of Maine, where the subtle influences of variable foodregimes and water temperatures could potentially exact largeinterannual differences in winter survival and recruitment tothe juvenile stage.     

Reproductive cycle and gonadal development of the Atlantic sea urchin Arbacia punctulata in the Gulf of Mexico: changes in nutritive phagocytes in relation to gametogenesis

ABSTRACT

The annual reproductive cycle comprises steady gametogenic activities that synchronize gonadal maturation and spawning rhythms, which are important for aquatic organisms including marine echinoderms (Echinodermata, Echinoidea). In this study, we report the annual reproductive cycle, gonadal development, and changes in nutritive phagocytes (NPs, which accumulate nutrients in germ cells) in relation to gametogenesis of the Atlantic sea urchin (Arbacia punctulata, an edible echinoid) in the Gulf of Mexico. Monthly changes in gonadal development and maturation were observed morphologically and histologically. We calculated gonadosomatic index (GSI) and compared the stages of gonadal development in order to determine the NPs index, and characteristics of germ cells (eggs and sperm) during the annual reproductive cycle. According to GSI and histological analyses, gametogenic activities were classified into four stages of both sexes: mature (June–August), spent (September–November), recovery (December–March), and growing (April–May). The GSI values in both sexes were high during summer months. In males, testicular lobules were densely packed with sperm from June to August. In females, however, mature eggs first appeared in some ovaries in May, numerically increased from June to July, and decreased in August. During gametogenesis, on the other hand, NPs in both testes and ovaries were depleted from June to August. Collectively, our results suggest that the Atlantic sea urchin spawns during summer months in the Gulf of Mexico. This is the first report, to the best of our knowledge, on gonadal development and changes in NPs during the annual reproductive cycle of any Arbacia species in the Gulf of Mexico.     

The annual gametogenic cycle of the sea anemone Metridium senile from the Gulf of Maine

A year-long study was conducted to quantify the reproductive cycle of the clonal sea anemone, Metridium senile, an important member of the benthic community in the Gulf of Maine (GOM). M.senile is an abundant sessile invertebrate that forms identifiable clonal aggregations that are maintained by pedal laceration but individuals can also reproduce sexually. Specimens were collected monthly from a shallow (< 10 m) subtidal site and the reproductive cycle of male and female anemones was described on a seasonal basis using histological preparation of the gonads and light microscopy. Gametogenesis was determined for both sexes and showed that females undergo spawning during the seasonal transition from summer to fall, while males have mature sperm within testicular cysts in the winter, spring, and summer. Environmental data recorded at the study site shows that the spawning period occurs during a period of peak water temperature (August-September), and rapid gametogenesis occurs during periods of high food availability (April). The low percentage of reproductively mature anemones and the dominance of asexual reproduction in this population are believed to be the result of the high flow regime at this site.     

Coral reproduction in the world��s warmest reefs: southern Persian Gulf (Dubai, United Arab Emirates)

Despite extensive research on coral reproduction from numerous geographic locations, there remains limited knowledge within the Persian Gulf. Given that corals in the Persian Gulf exist in one of the most stressful environments for reef corals, with annual variations in sea surface temperature (SST) of 12°C and maximum summer mean SSTs of 36°C, understanding coral reproductive biology in the Gulf may provide clues as to how corals may cope with global warming. In this study, we examined six locally common coral species on two shallow reef sites in Dubai, United Arab Emirates (UAE), in 2008 and 2009 to investigate the patterns of reproduction, in particular the timing and synchrony of spawning. In total, 71% colonies in April 2008 and 63% colonies in April 2009 contained mature oocytes. However, the presence of mature gametes in May indicated that spawning was potentially split between April and May in all species. These results demonstrate that coral reproduction patterns within this region are highly seasonal and that multi-species spawning synchrony is highly probable. Acropora downingi, Cyphastrea microphthalma and Platygyra daedalea were all hermaphroditic broadcast spawners with a single annual gametogenic cycle. Furthermore, fecundity and mature oocyte sizes were comparable to those in other regions. We conclude that the reproductive biology of corals in the southern Persian Gulf is similar to other regions, indicating that these species have adapted to the extreme environmental conditions in the southern Persian Gulf.     

The application of acoustic Doppler current profilers to measure the timing and patterns of coral larval dispersal

An experiment was conducted along the reefs off west Maui, Hawaii, during the summer of 2003 to monitor the spawning of the reef-building coral Montipora capitata and to determine the role of ocean currents in dispersing the larvae from the natal reef. Instruments documented the environmental forcing during the coral spawning season; drifters were deployed on three successive nights following direct observations of coral spawning. Both the timing and relative magnitude of the coral spawning were identifiable in acoustic backscatter data and correlated to plankton tow data. Each drifter track showed that the surface water containing coral eggs and planula larvae were transported rapidly offshore and not locally retained. Wind and current patterns during the previous year and during subsequent coral spawning events later in the summer were similar to those observed during the drifter releases. This suggests that the trajectories observed during the focused experiment are representative of the general pattern of larval dispersal off west Maui. These findings demonstrate the application of acoustic profilers for remotely imaging coral spawning and predicting their initial dispersal patterns.     

Evidence of Atlantic bluefin tuna spawning in the Bay of Biscay,north-eastern Atlantic

The spawning grounds of the Atlantic bluefin tuna (Thunnus thynnus) are traditionally considered to be the Gulf of Mexico (Gulf of Mexico) and the Mediterranean Sea (Mediterranean Sea). However, for the western Atlantic, unequivocal evidence of bluefin spawning outside the Gulf of Mexico has been shown. In this study we present the first records of genetically confirmed bluefin larvae in the southern Bay of Biscay (eastern Atlantic). These findings provide evidence of bluefin spawning activity outside the Mediterranean Sea, in the north-eastern Atlantic. However, our results suggest that the bluefin spawning in the Bay of Biscay is a sporadic phenomenon.     

Biochemical and energy dynamics throughout the reproductive cycle of the striped venus Chamelea gallina (Mollusca,Bivalvia)

The striped venus Chamelea gallina is an important commercial bivalve species in Europe. However, large inter-annual fluctuations in stock abundance and periodic recruitment failure threaten the biological and economic sustainability of this fishery. This study aimed to improve the knowledge of the reproductive cycle and reproductive strategies of this species from the Algarve coast (southern Portugal) in order to contribute to the establishment of management measures and to assess its potential for aquaculture. The reproductive cycle of C. gallina followed a seasonal cycle, significantly influenced by sea surface temperature and food availability. Gametogenesis took place in winter, coinciding with the phytoplankton bloom. Spawning occurred during summer, followed by a short period of sexual inactivity in autumn. Condition index did not reflect the reproductive cycle of C. gallina and generally, followed the same trend of chlorophyll a. Glycogen was positively correlated with gonadal index and chlorophyll a. High total lipid values were recorded throughout gonad ripeness and spawning, but decreased at the end of the spawning and in the rest period. The extended spawning period of C. gallina will allow larvae to be obtained for much of the year by artificial spawning of wild broodstock.     

Factors structuring reproductive habitat suitability of Engraulis encrasicolus in the south coast of Sicily

Egg and larval abundance, obtained from annual summer cruises carried out in the Strait of Sicily during 6 years (1997 to 2002), was used to identify the spawning and retention areas of the anchovy Engraulis encrasicolus . Larval amino acid composition was used to analyse the nutritional condition of the larvae collected during the 2002 survey. As the anchovy spawns along the south Sicilian coast, but mainly in the north‐west and middle parts of the coast, the higher number and larger size larvae in the south‐east end of the Sicilian coast indicated that larvae were transported by the main branch of the Atlantic‐Ionian Stream and retained in the south‐east end of the Sicilian coast (Cape Passero). This retention process, however, did not always favour the nutritional condition of the larvae, because the larvae were not transported to an area with a higher primary production or with a higher stability. In 2002 larval amino acid concentration was higher in the spawning areas than in those areas where the larvae were transported to, indicating a lower nutritional condition of the larvae in the nursery areas. The lack of enrichment and concentration processes, and low primary production in this area which limited body growth rate may explain the low locally reproducing anchovy population in the south coast of Sicily.     

A century of research on the larval distributions of the Atlantic eels: a re‐examination of the data

The spawning areas of the Atlantic freshwater eels were discovered about a century ago by the Danish scientist Johannes Schmidt who after years of searching found newly hatched larvae of the European eel, Anguilla anguilla, and the American eel, Anguilla rostrata, in the southern Sargasso Sea. The discovery showed that anguillid eels migrate thousands of kilometers to offshore spawning areas for reproduction, and that their larvae, called leptocephali, are transported equally long distances by ocean currents to their continental recruitment areas. The spawning sites were found to be related to oceanographic conditions several decades later by German and American surveys from 1979 to 1989 and by a Danish survey in 2007 and a German survey in 2011. All these later surveys showed that spawning occurred within a restricted latitudinal range, between temperature fronts within the Subtropical Convergence Zone of the Sargasso Sea. New data and re‐examinations of Schmidt's data confirmed his original conclusions about the two species having some overlap in spawning areas. Although there have been additional collections of leptocephali in various parts of the North Atlantic, and both otolith research and transport modelling studies have subsequently been carried out, there is still a range of unresolved questions about the routes of larval transport and durations of migration. This paper reviews the history and basic findings of surveys for anguillid leptocephali in the North Atlantic and analyses a new comprehensive database that includes 22612 A. anguilla and 9634 A. rostrata leptocephali, which provides a detailed view of the spatial and temporal distributions and size of the larvae across the Atlantic basin and in the Mediterranean Sea. The differences in distributions, maximum sizes, and growth rates of the two species of larvae are likely linked to the contrasting migration distances to their recruitment areas on each side of the basin. Anguilla rostrata leptocephali originate from a more western spawning area, grow faster, and metamorphose at smaller sizes of <70 mm than the larvae of A. anguilla, which mostly are spawned further east and can reach sizes of almost 90 mm. The larvae of A. rostrata spread west and northwest from the spawning area as they grow larger, with some being present in the western Caribbean and eastern Gulf of Mexico. Larvae of A. anguilla appear to be able to reach Europe by entering the Gulf Stream system or by being entrained into frontal countercurrents that transport them directly northeastward. The larval duration of A. anguilla is suggested to be quite variable, but gaps in sampling effort prevent firm conclusions. Although knowledge about larval behaviour is lacking, some influences of directional swimming are implicated by the temporal distributions of the largest larvae. Ocean–atmosphere changes have been hypothesized to affect the survival of the larvae and cause reduced recruitment, so even after about a century following the discovery of their spawning areas, mysteries still remain about the marine life histories of the Atlantic eels.     

A first glimpse of larval ecology of halibut species in the Gulf of St. Lawrence,Canada

Knowledge of the larval ecology of winter-spawning fish from the Estuary and Gulf of St. Lawrence, Canada, remains scarce due to the seasonal ice cover that prevents ichthyoplankton sampling using conventional methods. Two winter-spawning species, Atlantic halibut (AH, Hippoglossus hippoglossus) and Greenland halibut (GH, Reinhardtius hippoglossoides), support the most important groundfish fisheries of this area. In March 2020, the authors captured 10 halibut larvae ranging in size from 5 to 14 mm during an opportunistic survey in the GSL onboard an icebreaking vessel. Of these, eight were AH and two GH. Judging by their very small size, the larvae were only a few days old, suggesting that the spawning grounds are close to the capture sites. This effort constitutes a first step in validating the putative spawning areas for these two important GSL stocks. This knowledge is important for the conservation and sustainable management of these fisheries.