State-dependent spawning migration in Norwegian spring-spawning herring

Norwegian spring-spawning herring Clupea harengus winter in fjords of northern Norway, whereas the subsequent spawning occurs at various locations along the coast with a main bulk off the south-western coast. The distance of the southward spawning migration tends to increase with the length and condition of the fish. The costs and benefits of the southward migration were modelled in terms of fitness (number of surviving offspring). The model assumes that larvae have increasing growth and survival rates further south as they pass through warmer water during the northward drift in the coastal current. In agreement with the observed spawning distribution, optimal spawning grounds were predicted off the southwestern coast and farther south with increasing fish length and condition. The present study suggests that homing is not a successful strategy for these herring, and instead the selection of spawning grounds depends on individual internal state (length, condition), the cost of migration and the probability of larval survival.     

Differential utilization of energy during wintering and spawning migration in Norwegian spring-spawning herring

The energy utilization in non-feeding Norwegian spring-spawning herring Clupea harengus was examined during periods of wintering in a fjord system in northern Norway and southward spawning migration (800 km distance) to the main spawning area off western Norway. At the onset of wintering the gonadosomatic index (IG) was higher and the amount of mesenteric fat was lower in male than in female herring, whereas the subsequent increase in IG and decrease in mesenteric fat occurred at a higher rate in females than in males. However, no differences were recorded between sexes in fat content (%) of whole fish at any time of the year. The absolute energy loss increased, whereas the relative energy loss decreased, with fish length. Furthermore, the absolute and relative length specific energy losses were two to three and three to four times higher, respectively, during spawning migration than during wintering. The distinct energy loss was attributed mainly to fat metabolism, whereas the storage protein only was transferred from the soma to the gonads and remained relatively constant in whole herring throughout the wintering and spawning migration. © 1999 The Fisheries Society of the British Isles     

Interannual variations in condition indices of larval Norwegian spring-spawning herring

The sea water temperature off the Norwegian coast was lower in 1994 than in 1992 and 1993. RNA/DNA ratios of Norwegian spring spawning herring increased with increasing larval dry weight all years, except for larvae sampled south of 62°N in 1994. The RNA/DNA ratios indicated that each year, only a small portion ( 0.7%) of the larvae were starving. RNA/DNA ratios and temperature were negatively correlated in 1992, but in other years no significant correlations were found. Residuals of In RNA v . In DNA and In W v. L _s were poorly correlated in all years, but residuals of In RNA v. In DNA and In DNA v. In W were negatively correlated in all years. Principal component analysis showed that the RNA/DNA ratio and DNA (% of weight) were correlated with different axes. Abundance data for herring at the early larval and 0-group stages in 1992–1994 indicated higher mortality in 1994 compared with the other years. The data do not indicate that average larval condition was poorer in 1994 than in other years. However, the variability in larval condition was higher in 1994 than in other years, and the condition of later larval stages was relatively lower in 1994 than in other years.     

Spatially structured interactions between a migratory pelagic predator, the Norwegian spring-spawning herring Clupea harengus L., and its zooplankton prey

Spring-spawning herring Clupea harengus was patchily distributed over large parts of the Norwegian Sea in May 1995–2005, during the early phase of the annual feeding migration. Overall, herring tended to be found in areas with intermediate biomasses of zooplankton prey, intermediate water temperatures and relatively high salinities. Herring had more food in their stomachs in areas of relatively low water temperature and high herring abundance. Hydrographical conditions revealed that herring was feeding mainly within Atlantic water masses, and more intensely in western and northern regions of the Norwegian Sea. Zooplankton biomass was patchily distributed, and was generally higher towards the western parts of the Norwegian Sea. Here, zooplankton biomass in year _{i +1} was also negatively associated with herring spawning stock biomass in year _i , while there was no evidence for such an association in the eastern region; indicating that herring may have a geographically structured 'top–down' effect on the recruitment of its zooplankton prey. The fact that herring was not typically associated with the areas containing the greatest zooplankton biomasses may reflect that the fish had not yet reached the most profitable feeding grounds or alternatively that herring was depleting zooplankton biomass.     

Development stage distribution as a proxy for feeding success and growth for first feeding Norwegian spring spawning herring larvae

The estimation of growth rates in young herring larvae (Clupea harengus) in the field can be difficult because the primary increments in the otoliths may not be discernible or formed at a daily level. Likewise, the estimation of mortality rates of fish larvae in the field is very difficult to achieve, especially in a rigorous quantitative manner. In this study, the authors suggest the use of a stage-based proxy of feeding success, growth and potential survival or mortality risk of field-caught larvae. The stage-based proxy is derived based on observations from previous laboratory studies where larvae successfully completing start-feeding on external food sources will advance through the early development stages, whereas those that do not (unsuccessful larvae) remain and accumulate in the development stage preceding first feeding. The relative occurrence of larvae in the early development stages is therefore expected to reflect feeding conditions of the larvae, with higher ratios of unsuccessful larvae indicative of poor feeding success and higher mortality risk. Using field data on Norwegian spring spawning herring, the authors document that the relative occurrence of larvae in the late non-feeding stage is significantly higher at lower average zooplankton concentrations, in line with the predictions of the authors that this novel approach of using a stage-based proxy could be a useful indication of feeding success, growth and mortality in the field. Further, there was a significant interaction effect with ambient temperature, with the ratio being higher at low zooplankton concentrations at higher temperatures. This study also suggests that these findings are not population specific as the same accumulation of non-feeding larvae in the late non-feeding stage was observed in laboratory-reared larvae of both autumn and spring spawning herring populations.     

Feeding, Ichthyophonus sp. infection, distribution and growth history of Norwegian spring-spawning herring in summer

After its collapse in the late 1960s, 1991 was the first year of systematic herring investigations after the Norwegian Sea stock returned to the traditional summer-feeding areas. Of 11 prey groups three dominated the diet; Calanus finmarchicus, Parathemisto spp. and Sebastes spp. There was a high prevalence of Ichthyophonus sp. at stations in the central and western parts of the Norwegian Sea where the cpue was low. This suggests that infected individuals were unable to return from the feeding to the wintering area of the Norwegian fjords. Highest cpue were recorded between 68–73° N and 5–15° E. Age groups 2, 3 and 8 were most frequently caught and age group 8 (the 1983 year class) dominated the samples. Herring occurred in dense concentrations where C. finmarchicus dominated diet. 0-group fish dominated diet at stations along the coast from 66–73° N and out to c . 300 km off the coast. Mature herring overlapped in distribution with 0-group fish and Sebastes spp. contributed largely to herring diet in this area. The first rich year-class after the collapse, the 1983 year class, had a faster growth than rich year classes from past herring periods, but slower than poor ones.     

Predation by sprat and herring on pelagic fish eggs in a plaice spawning area in the Irish Sea

Predation on planktonic fish eggs was examined by stomach content analysis of sprat and herring sampled in a plaice spawning area to the east of the Isle of Man in March 1993. Plankton samples were taken to examine prey selection. The clupeids showed a selection for the later developmental stages of plaice eggs. Plaice eggs had a refuge in size from predation by sprat <80 mm total length which selected smaller non-plaice eggs. However, herring and sprat >80 mm total length showed a strong selection for plaice eggs, presumably due to their large size.     

Gonad maturation and spawning times of Siganus sutor off the Keny a coast: evidence for definite spawning seasons in atropical fish

The gonad maturation cycle of Siganus sutor (Valenciennes, 1835) (Osteichthyes-Siganidae) is described for both males and females using macroscopic criteria for the testes and both macroscopic and microscopic ones for staging the ovaries. Siganus suror has two major spawning seasons: one in January/February and the other in May/June. The presence of these seasons is established by (a) the temporal variations in the condition factor and in the relative weight of the gonads, (b) the progression of peaks of maturity stages with seasonal occurrence of spent fish in the samples, and (c) the seasonal appearance ofjuveniles. This is a significant result for a tropical marine fish.     

Effect of size on spawning time in the lesser sandeel Ammodytes marinus

Ovarian development was examined in relation to size and temperature in late pre‐spawning Ammodytes marinus over 5 years. Oocyte diameter was positively related to length indicating that larger females spawned earlier. Age and temperature, whilst accounting for the effect of length, were not found to affect oocyte development, although the thermal range examined was only 1·3° C.     

Intercalibration of hydroacoustic and mark–recapture methods for assessing the spawning population size of a threatened fish species

Hydroacoustic counting and a three-year mark–recapture study with passive integrated transponders (PIT tags) were used to estimate the size of a spawning population of nase Chondrostoma nasus , a threatened potamodromous cyprinid that undertakes annual spawning migrations into a tributary of the Danube River. In 2005, the estimates of the size of the spawning population from the hydroacoustic counts ( N = 2234, 95% CL 1929–2538) and from the Jolly–Seber model ( N = 1198, 95% CL 461–5842) corresponded well. Estimates from the jackknife-estimator based on the hydroacoustic counts yielded slightly higher values ( N = 2783, 95% CL 2529–3037), but were still in the same order of magnitude as those from the hydroacoustic and mark–recapture approach. At low run-size, hydroacoustic counting was more time consuming and technically demanding than mark–recapture studies. At the same time, it was non-invasive, provided real-time data on a fine temporal scale, and estimates showed less variability than the Jolly–Seber model. Mark–recapture of fish in spawning streams involved substantial disturbance at a sensitive stage of the life cycle. Hence, hydroacoustics is highly suited for population estimates of threatened potamodromous fishes, where interference needs to be minimized.     

Variation in egg mass within two Atlantic herring Clupea harengus stocks

The main objective of this study was to investigate if egg size (mass) at spawning is invariant for Scotia-Fundy summer and autumn (SFSH) and Icelandic summer (ISSH) spawning herring Clupea harengus. Oocyte dry mass measurements for SFSH females collected in 2001 and ISSH females collected in 1999 and 2000 showed a large variation. Difference in egg dry mass among fish was found to vary by as much as twofold in each stock. For ISSH, variation in egg mass was also apparent from oocyte volume measurements made jointly with a histological examination of the ovaries. Approximately 20% of the variation in egg mass could be explained by maternal whole-body mass or total length, indicating that length or age composition in the stocks can potentially influence the recruitment success. This implies that fisheries management strategies should aim to maintain a broad range in age composition.     

Promiscuous spawning behaviour of the tropical herring Spratelloides gracilis

Spawning behaviour of the blue sprat Spratelloides gracilis was observed at shallow reefs of Kuchierabu‐jima Island, southern Japan. The fish carried out promiscuous spawning behaviour on substrata covered with algal mats mainly in the intertidal region of a shallow inlet. The spawning events occurred only at predawn (20–50 min before sunrise) and daily for c . 25 days from mid‐June to mid‐July.     

Sexual maturity cycle and spawning of Greenland halibut Reinhardtius hippoglossoides in the Davis Strait

Female sexual maturation cycle and the main spawning time of Greenland halibut Reinhardtius hippoglossoides in the Davis Strait were studied through regularly collected samples during 1 year starting in spring 2003. Samples were collected from the southern slope of the Davis Strait Ridge between Canada and Greenland in the depth range 1000–1500 m. Female sexual maturation was described using different approaches: gonado‐somatic index, visual macroscopic maturity stage index, histological microscopic maturity index and oocyte diameter measurements. A significant increase in the gonado‐somatic index was seen from September onwards until February with a maximum estimated value of 18%. The proportion of mature fish increased from December until March. At the same time, the proportion of females with a low gonado‐somatic index also increased from February, indicating that spawning had occurred and females were recovering. Oocyte diameter distribution revealed a leading cohort development during autumn through to December to February. A coupling between sexual maturity and fish condition was seen for females in maturing condition indicating a steady build up of stored energy in the liver from June to November.     

Larval distribution, abundance, and stock structure of British Columbia herring

Pacific herring Clupea pallasi spawn in shallow and intertidal areas. Larval distributions and abundance of three major and several minor stocks were examined to determine if larvae mix among different spawning locations. The surveys were short, intensive, and occurred mainly within the first 30 days of larval life. Each major stock had a discrete larval distribution with continuous larval distributions within stock boundaries. Some overlap of distributions occurred among smaller stocks. Most larvae were found in inshore waters but there were no obvious oceanographic factors, such as fronts or eddies, to explain all the distributions. Estuarine circulation in inlets could promote larval retention in some areas. The results support the present geographic definitions for local herring stocks. The results are generally consistent with the concept of discrete larval distribution, including retention areas, as the basis for herring Stock Structure.     

Aggregation and spawning of the humphead wrasse Cheilinus undulatus (Pisces: Labridae): general aspects of spawning behaviour

The humphead wrasse Cheilinus undulatus formed resident spawning aggregations daily after high tide at specific locations along the seaward edge of the Palau barrier reef. The location and extent of one aggregation site remained consistent for 6 years with no physical features distinguishing it from adjacent areas. Spawning was documented most months and probably occurred year round with possible seasonal and lunar variation. Spawning males arrived first at the site, followed by females and potentially small primary males. The aggregation female to male sex ratio was estimated to be between 6:1 and 10:1. A maximum of 15 males and 100–150 females were observed at the site. A male courtship posture with the anal fin pointed, the caudal fin folded down and the dorsal fin folded against the body was maintained while swimming a few metres off the bottom in view of females. When ready to spawn females rose up as the posturing male passed and the pair released gametes in a relatively sedate fashion near the surface along the shelf break. No attempted predation on spawning adults was seen. Egg predation after spawning was uncommon. On days with early to mid‐day high tides the spawning period started 2·0–2·5 h after high tide when the speed of lagoon–ocean tidal currents peaked and lasted c. 1 h. On days with later afternoon high tides, spawning occurred sooner after high tide and before current speeds peaked. Other fishes with planktonic eggs spawned at the site as pairs or small groups in a rough succession after high tide with C. undulatus, the last species to spawn.     

Variation in body condition, fat content and growth rate of Icelandic summer-spawning herring Clupea harengus L.

Interannual variation in Fulton's condition factor ( K ), total body-fat content and growth rate were investigated for the Icelandic summer-spawning herring Clupea harengus over the period from 1963 to 1999. The results show a close relationship between K and the body-fat content in the autumn, both of which correlated to the growth rate in the same calendar year. A relationship was also found between K during the prespawning season (February to June) and K in the autumn (September to December) in the same year. The different measures of body condition were not found to be related to sea temperature or zooplankton indices, while the spawning stock biomass had negative effects on the fish condition. The results imply that the body condition in the autumn is representative of the feeding success over the whole foregoing year and is therefore related to the growth rate that year. The growth rate appears to be relatively uniform throughout the year.     

Norwegian spring‐spawning herring as the test case of piecewise linear regression method for detecting maturation from growth patterns

Data from Norwegian spring‐spawning herring Clupea harengus, were used empirically to assess the performance of a regression method aimed at detecting the time of the onset of maturation from growth trajectories. At the level of the whole dataset, the piecewise linear‐regression method was accurate and showed only a minor bias (?0·17 years) relative to age at maturation visually read from scales. The method, however, was relatively imprecise and provided an estimate of age at maturation equal to the one read from the scale in less than half of the cases (47·6%). Moreover, bias was strongly dependent on age at maturation: the age at maturation of early maturing fish was often overestimated, whereas the opposite was true for late‐maturing fish. Accuracy and precision of the regression method relative to visual readings also depended on the growth type (determined by the nursery area of young C. harengus) and cohort but not on sex. Modifying the original regression method enabled marginally improving the precision of the approach but a strong maturation age‐dependent bias persisted. The results with C. harengus suggested that age‐at‐maturation estimates from the piecewise linear regression method should be treated with caution.     

Oceanic migration and spawning of anguillid eels

Many aspects of the life histories of anguillid eels have been revealed in recent decades, but the spawning migrations of their silver eels in the open ocean still remains poorly understood. This paper overviews what is known about the migration and spawning of anguillid species in the ocean. The factors that determine exactly when anguillid eels will begin their migrations are not known, although environmental influences such as lunar cycle, rainfall and river discharge seem to affect their patterns of movement as they migrate towards the ocean. Once in the ocean on their way to the spawning area, silver eels probably migrate in the upper few hundred metres, while reproductive maturation continues. Although involvement of a magnetic sense or olfactory cues seems probable, how they navigate or what routes they take are still a matter of speculation. There are few landmarks in the open ocean to define their spawning areas, other than oceanographic or geological features such as oceanic fronts or seamounts in some cases. Spawning of silver eels in the ocean has never been observed, but artificially matured eels of several species have exhibited similar spawning behaviours in the laboratory. Recent collections of mature adults and newly spawned preleptocephali in the spawning area of the Japanese eel Anguilla japonica have shown that spawning occurs during new moon periods in the North Equatorial Current region near the West Mariana Ridge. These data, however, show that the latitude of the spawning events can change among months and years depending on oceanographic conditions. Changes in spawning location of this and other anguillid species may affect their larval transport and survival, and appear to have the potential to influence recruitment success. A greater understanding of the spawning migration and the choice of spawning locations by silver eels is needed to help conserve declining anguillid species.     

Maturation and spawning processes of anadromous and resident pond smelt in lake ogawara

Pond smelt,Hypomesus nipponensis McAllister, in Lake Ogawara demonstrate alternative life history strategies, as evidenced by the coexistence of anadromous and resident fish. However, it is unknown if anadromous and resident groups interbreed. In this study, maturation and spawning processes were examined and compared between anadromous and resident groups. Histological observations indicated negligible variation in the maturational stage composition of oocytes, the frequency of oocyte diameter being unimodal for all specimens at different maturational stages. Oocytes were absent in the ovaries of spent fish. Accordingly, the species can be considered a semelparous spawner with unimodal oocyte diameter distribution. Temporal changes in the proportion of spent fish were compared between anadromous and resident groups. Spawning of both groups began in late March and peaked over April 8–12. Although both groups did not differ significantly in the period of peak spawning, anadromous fish finished spawning earlier than resident ones. Anadromous fish were not able to spawn upon migration into Lake Ogawara, and quickly matured after immigration, contrasting with resident fish.