Size Of Spawners Research Articles

Shifting spawning phenology in Hudson River American Shad

AbstractObjectiveThe objectives of this study were to investigate potential shift in the phenology of spawning for the Hudson River American Shad Alosa sapidissima and to identify factors contributing to the decline of the Hudson River shad population.MethodsThis study utilized American Shad eggs collected from the Long River Ichthyoplankton Survey. Logistic models were employed to estimate spawning phenology metrics, including the onset, peak, cessation, and duration of the spawning season. Additionally, we investigated the effects of biotic and abiotic variables on spawning timing.ResultThis study suggests that temporal changes in the spawning onset exhibited high variation and were not significant. However, a significantly delayed spawning peak and cessation, along with an extended spawning season, were observed. Additionally, the findings of this study revealed a correlation between the timing of spawning onset and water temperature, indicating that a 1°C increase in water temperature was associated with a 3.66‐day earlier initiation of spawning. Smaller female spawner size was associated with delayed peak and cessation of the spawning season. With a 1‐cm decrease in the average female spawner size, the peak of the spawning season was delayed by 1.79 days and the cessation of the spawning season was delayed by 2.87 days.ConclusionThe findings indicate that water temperature may be a crucial trigger for initiating spawning in Hudson River American Shad, with higher water temperatures associated with earlier spawning onset. Moreover, the findings indicate that the timing of spawning peaks and cessations has been progressively delayed over time, and this is linked to the size of female spawners.

Multigenerational exposure to warming and fishing causes recruitment collapse, but size diversity and periodic cooling can aid recovery

Global warming and fisheries harvest are significantly impacting wild fish stocks, yet their interactive influence on population resilience to stress remains unclear. We explored these interactive effects on early-life development and survival by experimentally manipulating the thermal and harvest regimes in 18 zebrafish (Danio rerio) populations over six consecutive generations. Warming advanced development rates across generations, but after three generations, it caused a sudden and large (30-50%) decline in recruitment. This warming impact was most severe in populations where size-selective harvesting reduced the average size of spawners. We then explored whether our observed recruitment decline could be explained by changes in egg size, early egg and larval survival, population sex ratio, and developmental costs. We found that it was most likely driven by temperature-induced shifts in embryonic development rate and fishing-induced male-biased sex ratios. Importantly, once harvest and warming were relaxed, recruitment rates rapidly recovered. Our study suggests that the effects of warming and fishing could have strong impacts on wild stock recruitment, but this may take several generations to manifest. However, resilience of wild populations may be higher if fishing preserves sufficient body size diversity, and windows of suitable temperature periodically occur.

Shedding light on the link between the spatial distribution of eggs and survival in Northeast Arctic cod

AbstractSpreading the offspring in space and time may offer bet‐hedging benefits by buffering environmental influences on parts of the offspring distribution. It has previously been shown that high mean age and size of spawners in the Northeast Arctic stock of Atlantic cod (Gadus morhua) is positively associated with high abundance and wide spatiotemporal distribution of eggs. However, little support has been found for a wide egg distribution acting as a buffer against environmental influences by leading to higher mean or less variable recruitment. The lacking relation between egg distribution and recruitment can be due to data limitation or alternatively that wide egg distributions may be of limited relevance for survival in later stages. To investigate these two alternatives, we analysed the output from a mechanistic coupled physical–biological drift model that projected the abundance of juvenile cod on the nursing grounds in the Barents Sea from empirically estimated egg distributions. The projections were performed with initial egg distributions reflecting the expected distributions at high and low mean weight of the spawners. We found that the extended distribution at the fringes only marginally contributed to recruitment, as a large proportion of the extra eggs at high mean age and size of spawners did not reach suitable nursing areas or failed to grow to a suitable size by the end of the season. In conclusion, our results suggest that the buffering effect of a geographically wide distribution of spawning products is insignificant compared to other effects on recruitment.

Comparison of Reproductive Performance of Domesticated Litopenaeus vannamei Females Reared in Recirculating Tanks and Earthen Ponds: An Evaluation of Reproductive Quality of Spawns in Relation to Female Body Size and Spawning Order

Optimizing broodstock reproductive performance quality in aquaculture is crucial for planning long term genetic improvement programs to facilitate development of an effective seed dissemination strategy. In the current study, we investigated the relative reproductive performance of female Penaeus vannamei broodstock reared under two common rearing systems: i) recirculating tanks (RT) and ii) earthen ponds (EP), and evaluated relative individual female reproductive performance (RT vs EP), quality of reproductive females in relation to individual body size of spawners, and female reproductive quality relative to spawning order (number of spawning events per individual). No significant difference (P>0.05) was observed between RT-reared vs EP-reared females for: i) number of eggs per spawn (RT= 23.34 ± 0.72 × 104, EP= 22.45 ± 0.67 × 104), ii) number of nauplii per spawn (RT= 19.85 ± 0.85 × 104, EP= 19.53 ± 0.83 × 104), iii) hatch rate of eggs per spawn (RT= 0.83 ± 0.02, EP = 0.85 ± 0.02) or iv) relative fecundity - number of eggs per g of female (RT= 5.51 ± 0.15× 103, EP= 5.78 ± 0.19 × 103). We recorded 136 and 101 spawning events for RT and ER females, respectively. EP-reared females (1.93 ± 0.23) showed a significantly higher (P0.05). The data generated here will be used to optimize a genetic breeding strategy for our broodstock line and to develop a seed distribution strategy for the local production sector in China.

Ticket to spawn: Combining economic and genetic data to evaluate the effect of climate and demographic structure on spawning distribution in Atlantic cod.

Climate warming and harvesting affect the dynamics of species across the globe through a multitude of mechanisms, including distribution changes. In fish, migrations to and distribution on spawning grounds are likely influenced by both climate warming and harvesting. The Northeast Arctic (NEA) cod (Gadus morhua) performs seasonal migrations from its feeding grounds in the Barents Sea to spawning grounds along the Norwegian coast. The distribution of cod between the spawning grounds has historically changed at decadal scales, mainly due to variable use of the northern and southern margins of the spawning area. Based on historical landing records, two major hypotheses have been put forward to explain these changes: climate and harvesting. Climate could affect the distribution through, for example, spatial habitat shifts. Harvesting could affect the distribution through impacting the demographic structure. If demographic structure is important, theory predicts increasing spawner size with migration distance. Here, we evaluate these hypotheses with modern data from a period (2000–2016) of increasing temperature and recovering stock structure. We first analyze economic data from the Norwegian fisheries to investigate geographical differences in size of spawning fish among spawning grounds, as well as interannual differences in mean latitude of spawning in relation to changes in temperature and demographic parameters. Second, we analyze genetically determined fish sampled at the spawning grounds to unambiguously separate between migratory NEA cod and potentially smaller sized coastal cod of local origin. Our results indicate smaller spawners farther away from the feeding grounds, hence not supporting the hypothesis that harvesting is a main driver for the contemporary spawning ground distribution. We find a positive correlation between annual mean spawning latitude and temperature. In conclusion, based on contemporary data, there is more support for climate compared to harvesting in shaping spawning ground distribution in this major fish stock in the North Atlantic Ocean.

Effects of broodstock diets on growth of larval Atlantic cod (Gadus morhua L.)

A previous study using two commercial pelleted diets (broodstock and on-growing diets) and a baitfish diet to feed hatchery-reared Atlantic cod (Gadus morhua) broodstock revealed higher growth and size of spawners as well as higher fertilization and hatch success in the group fed the baitfish diet, but no differences in egg diameter or blastomere normality. In the present study, we explore differences in condition and growth of cod larvae to assess the effect of the abovementioned broodstock diets on offspring using protein-specific growth rates and RNA/DNA ratios (RNA/DNA). Changes in RNA/DNA with time (after 7 days post-hatch) were significantly lower in the broodstock diet group compared with the other two groups, suggesting a lower feeding condition and growth potential. However, our results show no differences in growth and survival up to 49 dph between the larvae originating from broodstock fed the three diets. Contrary to other authors’ findings, we found no correlation between the RNA/DNA and instantaneous growth rates possibly due to the variability in growth observed within diet groups. Our results provide information about the effects of broodstock diets on early larval growth and survival of Atlantic cod.

Incorporation of weight-specific relative fecundity and maternal effects in larval survival into stock assessments

Marine fish populations may exhibit a number of complexities in their reproductive dynamics, such as weight-specific relative fecundity (i.e. eggs produced per spawner weight increases with spawner size) and maternal effects (i.e. reduced larval survival with reduced spawner age), that are typically not incorporated in stock assessment models. Improved information on reproductive biology could alter estimates of stock productivity and potentially improve estimation of stock–recruitment parameters. In this study, we examined the influence of maternal effects and weight-specific relative fecundity on stock status (defined as reproductive potential and measured as eggs, larvae, or spawning stock biomass), Fmsy, and the statistical fit of stock–recruitment curves estimated within the Bering Sea/Aleutian Islands Pacific ocean perch (Sebastes alutus) and Gulf of Alaska walleye pollock (Theragra chalcogramma) statistical catch-at-age assessment models. In each example, estimates of recruitment strength and the variation around stock–recruitment curves were relatively insensitive to various indices of reproductive potential. However, weight-specific relative fecundity increased estimates of Fmsy, whereas maternal effects decreased estimates of Fmsy. The range of Fmsy estimates obtained for walleye pollock from eight indices of reproductive potential was 0.12–0.19, whereas the Fmsy range for Pacific ocean perch from three indices of reproductive potential was 0.079–0.084. For walleye pollock, weight-specific relative fecundity and recent increases in spawner weight-at-age resulted in an upward trend in reproductive potential relative to constant relative fecundity with spawner size. The two examples demonstrate the influence of reproductive biology upon stock productivity even in cases where residual recruitment variation is relatively unaffected, and motivate the ongoing monitoring of reproductive status and its incorporation in estimation of fishing rate reference points.

Size‐dependent reproductive success of wild zebrafish Danio rerio in the laboratory

Size-dependent reproductive success of wild zebrafish Danio rerio was studied under controlled conditions in the laboratory to further understand the influence of spawner body size on reproductive output and egg and larval traits. Three different spawner size categories attained by size-selective harvesting of the F(1)-offspring of wild D. rerio were established and their reproductive performance compared during a 5 day period. As to be expected, large females spawned more frequently and had significantly greater clutch sizes than small females. Contrary to expectations, small females produced larger eggs when measured as egg diameter with similar amounts of yolk compared to eggs spawned by large spawners. Eggs from small fish, however, suffered from higher egg mortality than the eggs of large individuals. Embryos from small-sized spawners also hatched later than offspring from eggs laid by large females. Larval standard length (L(S))-at-hatch did not differ between the size categories, but the offspring of the large fish had significantly larger area-at-hatch and greater yolk-sac volume indicating better condition. Offspring growth rates were generally similar between offspring from all size categories, but they were significantly higher for offspring spawned by small females in terms of L(S) between days 60 and 90 post-fertilization. Despite temporarily higher growth rates among the small fish offspring, the smaller energy reserves at hatching translated into lower condition later in ontogeny. It appeared that the influence of spawner body size on egg and larval traits was relatively pronounced early in development and seemed to remain in terms of condition, but not in growth, after the onset of exogenous feeding. Further studies are needed to explore the mechanisms behind the differences in offspring quality between large- and small-sized spawners by disentangling size-dependent maternal and paternal effects on reproductive variables in D. rerio.

Differential Egg Mortality of Georges Bank Cod and Haddock Inferred from Two Independent Estimates of Seasonal Egg Production

Georges Bank Atlantic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) stocks have undergone significant changes over the last 40 years with the reduction of older spawners and increased incidence of young mature fish. Captive studies, from other investigations, have noted that firstand second-time spawners exhibit lower egg hatching success, and especially for haddock compared to cod. Spawning stock biomass (SSB), derived from virtual population analysis (VPA), has been considered as an overestimate of a stock’s spawning potential since it does not fully account for differences in age or size of spawners. The Northeast Fisheries Science Center’s (NEFSC) ichthyoplankton Surveys (1979–1999) have provided an independent estimate of seasonal egg production to compare with a VPA-fecundity based estimate. Since the early stage eggs of the two species are indistinguishable, their numbers were estimated by apportioning the total egg abundance (cod plus haddock) at a station by the late stage abundance ratio, assuming equal mortality for the two species. For cod, the Surveys overestimated the VPA-derived egg production in many years by as much as an order of magnitude. However, for haddock, the VPA-derived egg production estimates were mostly higher than those from the Surveys. Comparing the ratio of the two egg production estimates versus the total combined VPA egg production of cod and haddock, the cod ratios were high when haddock was abundant, especially during the years 1995–1999, and closer to one when cod comprised the greater part of the total. The corresponding analysis for haddock was the reverse to that of cod further supporting the hypothesis that the higher egg mortality of the reduced age population of haddock biased the seasonal egg production estimates of both haddock and cod. These ratios appear to explain 60–70% of the variability in the ratio of Survey to VPA seasonal egg production, while other factors might be related to fecundity and age-class composition of the SSB, or changes in egg mortality rates over the time series. Our Survey estimates also suggest that haddock egg mortality is greater than cod. Thus, the seasonal egg production estimates from egg Surveys based on constant egg mortality are probably inaccurate, especially since the mid-1990s. Cod egg production may have been overestimated and haddock’s underestimated.

Reproductive Performance of Different Sized Wild and Pond‐reared Penaeus paulensis Females

AbstractThe present study analyzes the reproductive performance of three size classes of wild (average weight of 46.5, 55.0, and 63.2 g) and three size classes of pond‐reared (18.4, 21.3, and 25.1 g) Penaeus paulensis females. Unilaterally eyestalk ablated females were placed for 70 d in 3.6 m diameter tanks at 6 individuals/m2 and a 1.2:1 female‐male ratio. Number of spawns, eggs, nauplii, fertilization and hatching rates were recorded every second day. The length of unfed protozoea I larvae was also recorded. Larger broodstock shrimp tended to suffer higher mortality rates. Smaller, pond‐reared females started spawning at 28.2 ± 4.1 g. The number of eggs and nauplii per spawning event were not significantly different between experimental groups (P > 0.05), but due to their higher spawning frequency, small wild females (46.5 ± 3.7 g) outperformed medium (55.0 ± 1.8 g) and large ones (63.2 ± 7.0 g). No relationship between protozoea length and spawner size was found. Optimum size for the maturation of wild P. paulensis females was estimated to be from 30 to 60 g. It is suggested that the use of this size range can potentially enhance nauplii production of P. paulensis.

Restoration of an Interior Lake Ecosystem: the Kootenay Lake Fertilization Experiment

Abstract Kootenay Lake, B.C. (395 km2), has experienced a series of major perturbations in the past 50 years, resulting in the 1980s’ collapse of South Arm kokanee salmon stocks and a dramatic decrease in the abundance of North Arm stocks. Historical data indicate the collapse was due to reservoir construction together with subsequent impoundment and nutrient retention on both main inflow tributaries (Kootenai and Duncan rivers) to Kootenay Lake, combined with the introduction of an exotic mysid shrimp which is an efficient competitor with kokanee for zooplankton. Nutrients (47.1 t of P and 206.7 t of N) are now being added annually to the 174 km2 North Arm of Kootenay Lake (271.3 mg-m-2 P and 1,190.5 mg-m-2 N) in a 5-year (1992-96) experiment with the goal of restoring historical kokanee salmon biomass and productivity. The concentration of total phosphorus in the North Arm currently ranges from 4–10 µg-L-1 P, which indicates oligotrophic to mesotrdphic conditions, whereas the concentration of dissolved inorganic nitrogen remains in the oligotrophic range (i.e., <200 mg L-1 N) throughout the year. The seasonal mean zooplankton density observed in 1994 was higher than in 1992 and 1993, and similar to the highest densities observed from 1972 to 1984. Cladocerans comprised about 7.5% of the zooplankton in the North Arm in 1994, 12% in 1993 and 7.5% in 1992 as compared with less than 5% between 1949 and 1991. Combined North Arm kokanee escapement (Lardeau River and Meadow Creek) was 1.25 million fish in 1994 and was the highest observed since 1986, but lower than some escapements observed in the 1970s. Kokanee spawner size and fecundity have also increased. No significant trends have been observed in the Gerrard rainbow trout population. To date, the fertilization experiment has been successful at increasing the abundance of clado-ceran zooplankton and restoring kokanee populations in the North Arm of Kootenay Lake.

Simple Size-Structured Models of Recruitment and Harvest in Pacific Salmon (Oncorhynchusspp.)

Growing evidence suggests that in Pacific salmon (Oncorhynchus spp.), components of reproductive success besides fecundity are size dependent. However, managers setting escapement goals usually estimate reproductive potential of a stock in terms of total number of spawners, number of female spawners, or potential egg deposition given a mean size of spawners. Interannual variation in size composition of spawners may thus result in errors in assessing reproductive potential. Here, we develop models of recruitment and harvest and determine optimal size-selective harvesting strategies. These optimal strategies range from the current large-fish harvests (larger-than-average fish are caught, leaving smaller fish to spawn) to small-fish harvests, where the reverse is true. If the body mass (M) of individual spawners increases more rapidly than reproductive success (RS) with respect to increasing length of adult females (e.g., if number of eggs alone best measures RS), then sustainable biomass yield is maximized by the current large-fish harvest. However, if RS/M increases with increasing length of females (e.g., if total biomass of eggs best measures RS), then small-fish harvest maximizes sustainable yield. Evidence suggests that some salmon populations show this latter pattern; thus, large-fish harvests may generate suboptimal yields.

Spotted Seatrout Distribution in Four Coastal Louisiana Estuaries

We analyzed experimental gill-net catches of spotted seatrout Cynoscion nebulosus at 28 stations in various estuarine habitats over 3 years (1988–1990). Cluster analysis indicated that the 28 stations reduced to three “natural” groupings representing salinity zones that correspond to upper (oligohaline, 0–9‰), intermediate (mesohaline, 10–14‰), and lower (polyhaline, 15–30‰) estuarine zones. Principal components analysis applied to log-transformed gill-net catches yielded a two-factor model that explained 78% of the variation in the data; factors were interpreted as recruit size (females smaller than 31 cm in total length and males smaller than 26 cm) and spawner size (females larger than 31 cm and males larger than 26 cm). Both recruit and spawner models were highly significant (P < 0.0001), and the zone-by-season interaction accounted for the greatest proportion of the variation in the models (P < 0.0001). Abundances of recruit and spawner spotted seatrout were greatest in the lower estuarine zone (15–30‰) during the spawning season (May–August), when both recruit and spawner abundances had high positive correlations with salinity (P < 0.01). After the spawning season (September–December), spawners were uniformly distributed over all estuarine zones, whereas recruits were significantly more abundant (P < 0.01) in the upper estuarine zone (0–9‰). During this time, recruit spotted seatrout abundance showed a high negative correlation with salinity (P < 0.01).

Aspects of the spawning of labrid and scarid fishes (Pisces: Labroidei) at Enewetak Atoll, Marshall Islands with notes on other families

Spawning of 32 species of Labridae and 13 species of Scaridae was seen at Enewetak Atoll, Marshall Islands. Most spawned on a reef bisecting the main ocean-lagoon passage which had strong tidal currents. Others spawned on lagoon reefs and in Halimeda beds. Polygynous haremic, lek-like and promiscuous mating systems were found which were species specific. Data on reproductive patterns, sexual dichromatism, sexual dimorphism, seasonality and spawning behavior were determined. Many spawned during the day in a time-phase dependent pattern from near sunrise to sunset. Scarid spawning began at slack high water or after when currents were starting to move out of the lagoon. Labrid spawning usually started about 30 min later with some continuing up to 2 h after high tide. With high tide before sunrise, scarid spawning began 30–50 min after sunrise as the current started flowing over the reef. With high tide near sunset, spawning occurred with an incoming current. Most labroids spawned on all phases of the moon. Acanthuridae (6 spp), Caesionidae (1 sp.) and Zanclidae (1 sp.) spawned after high tide at the same time as labroids. Pomacanthidae (5 spp.) spawned only shortly before sunset without reference to tidal currents. Fishes producing pelagic eggs at the lagoon-ocean channel spawned (1) at or slightly after high tide (44spp.), (2) in late afternoon without reference to tide (6 spp.) or (3) after slack low water (1 sp. ). Spawning style can vary within a single species in different environments. Despite the presence of many piscivores, no successful predation on spawning adults was seen. Predation on newly released eggs was uncommon. Labrichthys unilineata and Anampses twistii attempted to defend their eggs for a few sec after release. Attacks by piscivores on spawning adults on tropical reefs occur once per 100–1000 spawnings. Most are wary when preparing to spawn and prespawning behavior is easily interrupted. The risk from piscivores goes down and spawning ascent speed decreases with increasing size of spawners. Egg predation by zooplanktivores is less for pair spawners than group spawners possibly due to less conspicuous gamete clouds and times of spawning. Increasing height of egg release, speed and length of the spawning ascent, and trajectory alteration of ascending adults are believed to make it more difficult for zooplanktivores to locate eggs after release. For labrids, permanent full sexual dichromatism was found among haremic, lek-like and promiscuous mating systems. Species with temporary full dichromatism, permanent and temporary partial dichromatism and monochromatism were haremic. Smaller scarids were believed to have lek-like, and larger species haremic, mating systems. Smaller scarids had male looping behavior and post spawning displays, plus faster spawning ascents and different locations for egg release than larger ones. Eggs of 21 labrids were spherical or nearly spherical, ranging from 0.55–0.80 mm in diameter, and most had one oil globule. Among 7 is scarids, 6 had spindle-shaped eggs ranging from 1.25 ×0.50 mm 2.14× 0.48 mm while one had a nearly spherical egg. One scarid egg lacked an oil globule.

Migratory dynamics and characteristics of breeding pink salmon (Oncorhynchus gorbuscha) from the Carp River, eastern Lake Superior

The pink salmon (Oncorhynchus gorbuscha) inhabiting the Great Lakes are unique to their species, the completion of their life cycles occurring entirely in fresh water. This report describes the breeding migration and characteristics of spawners from the Carp River, an eastern Lake Superior tributary 70 km north of Sault Ste. Marie, Ontario. Movement into the stream began at dusk each evening in 1983, seemingly in response to decreasing light levels. Nightly catch numbers varied through the 1983, 1984, and 1985 runs with date, river discharge, wind-generated turbulence, and water temperatures, although the influence of these factors differed with sex and season. Spawners varied in size through the runs each year but not in the same fashion. Spawner size and condition varied yearly in apparent response to prey abundance. Gonad maturation was complete on stream entry more often in males than in females, though this difference was less pronounced further upstream or after time spent in the river. Degree of secondary sexual character development, complete on stream entry in both sexes, differed in even- and odd-year spawners in relation to condition. The recovery rates of spawned-out males and females did not differ. Tagged fish wandered from the Carp River at a rate of at least 7%, many to spawn in adjacent streams. Despite between-year differences in some parameters, much of the breeding ecology of these fish remains comparable to that of anadromous pink salmon.

Relation between size‐at‐maturity and survival of progeny in chum salmon, Oncorhynchus keta (Walbaum)

Data on age‐ and size‐at‐maturity, growth, and abundance of chum salmon were collected from 1959 to 1977 at Olsen Creek in Prince William Sound, Alaska. Age composition of spawners (3 to 6‐year‐olds) varied from year to year: 4‐year‐old fish were the dominant age group in most (16 out of 19) years and 6‐year‐old fish usually represented less than 1 % of the returns. Mean size of older spawners was significantly larger than that of younger spawners. Size‐at‐maturity was similar among fish from different broods maturing at different ages in the same year. Size‐at‐maturity and survival of progeny were significantly related. The larger the mean size of spawners, the higher the survival rate to adulthood of their progeny. Possible reasons for this relationship are discussed.

Temporal and spatial segregation of spawning in sympatric populations of Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L.

Based on data from Norwegian streams with sympatric populations of Atlantic salmon and brown trout, it is suggested that temporal segregation is the main mechanism segregating Atlantic salmon and brown trout during spawning. Peak spawning of trout was about 15 days earlier than that of salmon. Physical factors, such as water depth, water velocity and distance from the river banks segregate spawning sites of salmon and trout poorly. Gravel sizes of the redds of salmon and trout were significantly different, though with a considerable overlap, and mean egg depth of salmon and trout were 0.18 and 0.12 m, respectively, probably attributable to the different size of spawners of salmon and trout. None of the temporal or spatial parameters analysed segregate spawners of salmon and trout completely. Species determination of eggs and alevins from the redds showed no interspecific superimposition of redds. It is, therefore, concluded that low survival of hybrids after hatching does not explain the low frequency of hybrids observed in sympatric populations of salmon and trout.

アユの産卵生態-III

On the depth of water of the spawning grounds of the Ayu, several observations have been reported, but there are noticeable discrepancies among them and the reason for the discrepancies has not yet been pursued thoroughly. The present study was undertaken to secure information on this problem with special regards to the relationships between the size of spawners and the depth of water of the spawning grounds. The study consists of observations carried out in the Sagami River in Kanagawa Prefecture and experiments performed in rearing ponds of the Freshwater Fisheries Research Laboratory at Hino, Tokyo. In the field observations, the depth of water of five representative spawning grounds in Sagami River and the size of spawners sampled there were measured. In the experiments, artificial spawning beds of different depths were prepared in the ponds and the depth of the beds where spawners of different sizes laid their eggs were determined (Fig. 1). The results obtained were as follows; 1) At natural spawning grounds, large spawners tend to gather about deep streams, and small spawners shallow streams (Table 1). 2) In experimental ponds, small spawners less than 13.6cm in length spawned exclusively on beds 50cm or less in depth. While large spawners more than 15.Ocm exercised no definite selection on the depth of spawning bed (though they often spawned on deep beds more than 50cm in depth). 3) Besides the depth of water, the size composition of bottom gravel has a significant effect on the spawning of the species, small spawners tend to select bottom of smaller gravel as their spawning beds than large spawners; generally in spawning grounds of the “shallow stream type” the current is less rapid and the gravels are smaller than in the case of the “deep stream type.” Accordingly it may be said that shallow streams have conditions suitable to requirements of small spawners, and vice versa (ISHIDA, 1961).

Survival of Brook Trout from Egg to Fingerling Stage in Two Michigan Trout Streams

Survival of wild brook trout (Salvelinus fontinalis) from the egg to the end of the first summer of life was investigated at the Hunt Creek Trout Research Station, Montmorency County, Michigan, during 1943-48, by confinement of spawners of known size and number in screened diversions, followed by direct enumeration of the resulting fingerlings. The numbers of eggs deposited were estimated from size of spawners. Percentage survival observed in six diversion experiments ranged from 4.5 to 31.7. Survival from egg to fingerling was estimated also for unconfined, natural populations in the Pigeon River and in Hunt Creek (the natural channel) during 1949-58. Consecutive fall population estimates gave the number of breeders present one fall and the number of fingerlings surviving to the following fall. Egg production for the population of breeders was estimated from egg-count data. From a series of 12 population estimates, survival rates from egg deposition to the following September ranged from 2.7 to 6.7 percent (significantly lower than in the diversion experiments at Hunt Creek). For a total of 24 observations (from confinements in diversions at Hunt Creek and Convict Creek, plus population studies on the Pigeon River and Hunt Creek), the percentage survival from egg to fall fingerling ranged from 2.7 to 43, and averaged 8.6. Excluding what appear to be unduly high results from four of the observations, the survival rates ranged from 2.7 to 8.8 percent (average, 4.7).

アユの産卵生態-II

It is well known that the Ayu spawn on river beds of gravel and their eggs adhere firmly to the pebbles, but little information is available on the size of gravels on which the eggs are laid and the requirements of spawners for the size composition of gravels of spawning bed. The present study is undertaken to these problems. For this purpose, on one hand, the size of gravels sampled from natural spawning beds in two well-known spawning grounds of Sagami River in Kanagawa Prefecture were measured and the numbers of eggs attaching to the gravels of various size were counted. On the other hand, the relationships between the size of spawners and that of gravels on which they spawn were pursued in experimental ponds (Fig. 1). The results obtained were as follows: 1) At the natural spawning grounds, most of the eggs were laid on small gravels 10mm or less in diameter (Table 1). 2) It was commonly observed in the ponds that the spawning fish measured 13.3-15.9cm in length flocked to the artificial spawning beds made of relatively small gravels, viz., 20mm or less in diameter, and spawned there. Among them the larger spawners occasionally laid their eggs on larger gravels 10-20mm in diameter, but the smaller ones exclusively spawned on smaller gravels 10mm or less in diameter (Table 2). 3) From these observations it is assumed that the Ayu tend to gather for spawning round the stream bed riched with small gravels and spawned there, and it is more clearly observed in smaller fish. The fact mentioned above seems to relate to the spawning behaviour of the fish. The Ayu have a habit of moving gravels by their movement in spawning, accordingly it is assumed that they prefer to beds of gravels which they can move compartively easily.