Peak Spawning Period Research Articles

Induced ovulation of female chinook salmon ( Oncorhynchus tshawytscha) at a production hatchery

A simple, low-cost method for inducing ovulation in hatchery-held adult chinook salmon was developed. Use of this method resulted in an increase in the number of eggs collected, by circumventing losses due to prespawning mortality. One hundred and twenty 4-year-old chinook salmon which had returned to the Capilano hatchery prior to 1 October were used as a stock population. Three weeks before the peak spawning period, 36 fish were divided into two groups. The control group received two injections of 0.65% saline spaced 3 days apart. The test group received a 0.1-mg/kg SG-G100 (salmon gonadotropin) injection, followed on the third day by a 2.5-mg/kg SG-G100 injection. By Day 8, 73% of the treated group and 0% of the control group had ovulated. Prespawning mortality accounted for 22% of the treated fish and 83.4% of the control fish. Treatment resulted in a net gain of 16,000 eggs over the control. The second experiment began 3 days before the peak spawning period. The 32 fish which remained in the stock population were divided into three groups and received two injections 3 days apart. The two treatment groups received either 0.1 mg SG-G100/kg followed by 2.0 mg SG-100/kg, or 0.1 mg SG-G100/kg followed by 50.0 mg salmon pituitary extract (SPE)/kg. The control group was injected with 0.65% saline. By Day 8, 73% of the SPE group, 82% of the SG-G100 group, and 40% of the control group had ovulated. Treatment resulted in a net gain of 16,000 eggs for the SG-G100 group and 19,000 eggs for the SPE group over the control. No significant differences in egg size or percent survival to the eyed stage were found between test and control groups. The misjudgment of maturity led to the stripping of several fish before their eggs were completely ovulated. The factors leading to these partial ovulations are discussed.

Growth and Mortality of Stolothrissa tanganicae

Stolothrissa tanganicae, an endemic clupeid of Lake Tanganyika (East Africa), dominates ring-net and traditional fisheries in most years, cycling from a low in catches in the first half of the year to high abundance in the latter half. From fish sampled in the ring-net fishery and from total catches of Stolothrissa we developed length frequencies by month, assessed monthly growth from modal lengths, and used Ford-Walford plots to assess a yon Bertalanffy growth constant k of 0.21 and maximum length (L∞) of 90 mm. From a length-frequency distribution, calculated monthly age frequencies, and a calculated age frequency in the catch, instantaneous mortality (Z) = 0.43 per month. Peak spawning in Zambia to the south takes place August-December, and from our data on monthly age-class contributions and published sources we concluded that the peak spawning period for Stolothrissa occurs January through April in the northern part of the lake, although recruits of other time origin may appear. A similar lag in biomass increase is probably adapted to timing of wind-induced nutrient upwelling and plankton production.

Seasonal changes in plasma cortisol, testosterone and oestradiol-17β in the plaice, Pleuronectes platessa L

Competitive protein binding and radioimmunoassay methods are described for the measurement of cortisol, testosterone and oestradiol-17β in plasma. Monthly plasma samples were collected from adult female, adult male and immature plaice. The low, summer gonadosomatic index of females increased from September until peak spawning time in March. In females, the concentrations of oestradiol-17β and testosterone began to rise in July and December and reached a February peak of 16.2 ± 1.0 and 6.2 ± 0.4 μg/100 ml respectively. By March concentrations had fallen rapidly to a low of 0.32 ± 0.02 μg oestradiol-17β/100 ml and 0.29 ± 0.13 μg testosterone/100 ml and remained low until July and December. Males showed similar but less distinct seasonal changes in the gonadosomatic index and steroid concentration. Testosterone levels were significantly higher in December (12.2 ± 1.5 μg/100 ml) than in either October (2.8 ± 0.3 μg/100 ml) or June (0.10 ± 0.02 μg/100 ml). The low levels of oestradiol-17β in males showed small significant seasonal changes. In immature plaice, plasma concentrations of testosterone and oestradiol-17β remained lower than 2.0 and 1.0 μg/100 ml throughout the year and showed no significant seasonal changes. Cortisol levels reached a maximum during the peak spawning period, not only in females (15.5 ± 1.1 μg/100 ml) but also in the nonbreeding immatures (18.2 ± 0.5 μg/100 ml). By June cortisol levels had fallen markedly in postspawned females (2.3 ± 0.4 μg/100 ml), postspawned males (3.7 ± 1.5 μg/100 ml) and in immatures (2.1 ± 0.5 μg/100 ml). Low cortisol levels persisted in all three groups until autumn. Plasma cortisol levels are at a high during the season of poor feeding when mobilisation of energy reserves is likely to be greatest.

Availability, Size Composition, Age Composition, and Growth Characteristics of Pacific Ocean Perch (Sebastes alutus) off the Northern Washington Coast During 1967–72

Cruises were conducted off the northern Washington coast each year during 1967–72 to study trends in the availability and biological characteristics of Pacific Ocean perch (Sebastes alutus).There were marked differences in catch rate between cruises, and corresponding differences in age composition and age–length structure indicated that they resulted from differential availability of an old, slow-growing segment of the population to on-bottom trawling. Females from this segment of the population were particularly available when the cruise coincided with the period of peak spawning, and both sexes were available during the 1972 summer cruise.Within cruises, adults from different portions of the bathymetric range showed similar age composition, but there was an inverse relationship between depth and size attained by age 15. Size compositions consequently contained a smaller proportion of large fish as depth increased.Juveniles were confined to the shallow portions of the bathymetric range. There were obvious between-year changes in the length and age composition of juveniles, caused by the temporal progression of a dominant 1961 year-class.Fluctuations in availability make it difficult to monitor changes in mortality and since age composition also varies with availability, the problem is compounded further. Unless all segments of the population are located and sampled in proportion to their abundance, estimates of mortality can be made only between those cruises where the old, slow-growing component of the population is equally available.

Distribution, Abundance, and Spawning of Yellowtail Flounder, Limanda ferruginea, in the Newfoundland Area of the Northwest Atlantic

Yellowtail flounder were found as far north as the Strait of Belle Isle; however, the main concentrations were located on Grand and St. Pierre banks. The highest concentrations were found at 31–35 fath (57–64 m) and temperatures of 3.1–4.8 C. Abundance on the Grand Bank apparently increased from 1961 to 1968 coincident with a general upward trend in bottom temperatures and a drastic reduction in the haddock population. The peak spawning period was the latter half of June. Fifty percent of the males were sexually mature at age 5 and 31 cm, and 50% of females at age 6 and 37 cm.

Sexual Maturity and Spawning of the American Plaice, Hippoglossoides platessoides (Fabricius), from Newfoundland and Grand Bank Areas

The peak spawning period of American plaice was the first part of April for Flemish Cap and ranged from the end of April for the Grand Bank and surrounding areas to the first part of June for Labrador–Northeast Newfoundland Shelf. Average bottom temperatures during spawning ranged from about 3.5 C for Flemish Cap and the deep spawning areas to the north, to about −1.3 C for St. Mary's Bay and the northern half of the Grand Bank. For the Southeast Grand Bank, Southwest Grand Bank, and St. Pierre Bank average spawning temperatures were 1.1, 2.8, and 2.7 C, respectively.There was evidence that some older, larger fish spawned first and also that plaice in deep water spawned later than those from shallower depths. At the latter depths light was a possible factor.According to samples from St. Mary's Bay the development of the sex products for spring spawning was apparently controlled by water temperatures during the previous fall and late summer.Fifty per cent of the females from Flemish Cap were mature at 7.8 years, the bottom temperature being almost constant at 3.5 C. For other areas the number of years ranged from 12.2 for the Southwest Grand Bank, with prevalent temperatures 0–3, to 15.2 C for St. Mary's Bay where temperatures were −1 to 1.0 C. For males the range was from 5.3 years for the Southeast Grand Bank to 7.5 years for St. Pierre Bank, no data being available for Flemish Cap.

The sexuality and spawning of Manx Pectinids

The results of a comparative study of the breeding biology of the Manx pectinids with special reference to Chlamys varia, C. distorta, C. tigerina, C. striata and C. furtiva are presented.It is shown that C. varia and C. distorta are protandric hermaphrodites with separate sexual phases similar to those of oysters, and evidence is presented on the possibility of sex reversal. The other species, C. tigerina, C. striata and C. furtiva, are truly dioecious, possessing at no stage germ cells of both sexes in the same gonad and as most individuals produce only one brood, the possibility of sex reversal is excluded.C. distorta has two large-scale or peak spawning periods, once in summer and for the second time in autumn, although some spawning occurs as a rule in most months of the year. The breeding behaviour of this species is intermittent and therefore the breeding period is prolonged.C. varia has two mass spawnings in a year, once in June and for the second time during September and October and the breeding period is not markedly different from that observed in French waters. Spawning is usually completed within a short period. C. tigerina and C. striata spawn only once annually in June and August respectively and most individuals have only one breeding cycle during their life time. C. furtiva however, may have two spawnings in a year, but the material available was inadequate to confirm this.