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doi: 10.1093/icesjms/36.2.106pmid: N/A
Electric fields affect large fish more strongly than small fish. Electrical fishing systems should preferentially capture the larger members of a fish stock, but in practice their selectivity is controlled by the uniformity of the electric field distribution and the particular fish reaction to electrical stimulation being exploited. From published data, the selectivity of a practical system is evaluated for various electric field distributions and types offish reaction. It is found that the probability of capture by the system increases with fish size, but with a high electric field strength the system can be non-selective.
doi: 10.1093/icesjms/36.2.110pmid: N/A
The annual breeding cycle of Raja clavata in British waters is described. The ovarian cycle starts in December with the appearance of small, white ovarian eggs. Copulatory activity, based on the evidence of clasper wounds in the cloaca of one female, starts in February and reaches a peak in May-June, based on the occurrence of spermatophores in the nidamental glands. Egg laying starts in February, reaches a peak rate in June and ceases by September. Two methods of estimating the annual egg production are described, one based on the proportion of mature female rays with egg capsules and observed rates of egg laying and the other based on the decline in the number of ovarian eggs. Although the latter method is only valid for the period June to September, for these months it gives similar results to those of the alternative method. The estimated average production is 140 eggs a year. The number of ovarian eggs present in April and May equals the number of eggs laid after 1 June. Taking account of egg production before 1 June gives the relationship between annual egg production (y) and total maternal length in centimetres (x) as y = 1.19.x + 25.1 over the length range 77 to 105 cm. Although there is evidence that the duration of the breeding season may vary both annually and geographically, the limited data show that variations in annual egg production from year to year are small. It is suggested that length of daylight is the main factor controlling the ovarian cycle and that temperature controls the rate of egg laying. The length/fecundity relationship is linear and the reasons why it does not follow the more usual cubic relationship are discussed.
doi: 10.1093/icesjms/36.2.119pmid: N/A
Data are presented on the spawning of whiting in captivity. The females were found to have a high fecundity, each shedding numerous batches of eggs over a spawning period that lasted for many weeks. Batch size tended to diminish throughout the spawning season. Measurements of the diameters and dry weights of eggs collected at different times from the commencement of spawning showed that eggs shed at the beginning of the season were both larger and heavier than those shed later. The possible significance of this phenomenon is discussed. It is shown that spawning imposes a greater physiological strain on females than on males. It was estimated that spawning caused losses in the body weight of females that ranged from 0.03% to 0.40% per day.
Talbot, J. W.; Boon, Myrtle J.
doi: 10.1093/icesjms/36.2.128pmid: N/A
Experiments have been carried out to investigate the adsorption of rhodamine-B 200 and rhodamine-WT on to sediments suspended in sea water. The results lead to a modified form of the equation for adsorption derived earlier by Talbot and Henry (J. Cons. int. Explor. Mer, 32: 7–16) although the overall adsorption loss agrees substantially with that derived earlier. Rhodamine-WT showed a smaller loss than rhodamine-B 200, the difference depending on suspended load and dilution.
Alverson, Dayton L.; Carney, Michael J.
doi: 10.1093/icesjms/36.2.133pmid: N/A
A series of growth and decay curves have been studied to interpret the effects of changing natural mortality and growth on the time at which a cohort will maximize the amount of material produced and the expected length of the time envelope. A graphic approach is used to familiarize marine biologists and students of fish population dynamics of factors governing the growth and decay of fish cohorts. Equations have been established to compute the age or time a cohort will achieve its maximum weight, the maximum expected weight of a cohort per given input, the weight of a unit number of animals at the critical size; and an equation has been established for forecasting the critical size for various values of M and K. If the coupled M, K1 values for any curve are equal and W∞1 is held constant, the maximum biomass generated by such cohorts will be constant, regardless of the coupled M, K values. Similarly, for all curves in which M equals K and W∞ is held constant, the critical size for all such curves will be identical. It was also noted that the maximum biomass generated per given input would be the same for all curves for which the M, K ratios were equal (assuming W∞, is held constant). The authors finally propose a method of making a first approximation of natural mortality based on the time a cohort will maximize its aggregate weight and reach its “theoretical” age maximum.
doi: 10.1093/icesjms/36.2.144pmid: N/A
A theoretical investigation of a number of stock and recruitment curves was made. A distinction was drawn between density-dependent and stock-dependent processes. The Beverton and Holt curve with density-dependent mortality yields only an asymptotic curve of recruitment on stock, whereas the Ricker curve is dome-shaped because a stock-dependent mortality is used.The role of density-dependent growth was also examined. The relationship of stock (S) and recruitment (R) of the form R = AS exp (–BS) was derived by Ricker and by Beverton and Holt (in a second equation) in different ways. Mortality in both derivations is stock-dependent. Perhaps by means of density-dependent growth, both stock and density processes can be combined to yield a dome-shaped curve.
doi: 10.1093/icesjms/36.2.150pmid: N/A
During their life histories fish grow up through the trophic levels of the marine ecosystem and it can be argued that their mortality due to predation is a density-dependent function of age. Such a function is derived formally and is applied to the plaice; a curve of mortality was calculated from larval life to the critical age of sixteen years, at which the specific growth rates and mortality rates become equal. For older fish an attempt was made to show that a senescent mortality takes place, i. e. after the critical age. In a study of stock and recruitment, if the trend of natural mortality with age can be established, then cohorts can be considered to have replaced themselves on average by the critical age. Therefore replacement is contained within the system as it must be and there is no need to establish a replacement stock, as for example in the Ricker curve.
Johnson, Allyn G.; Utter, Fred M.; Hodgins, Harold O.
doi: 10.1093/icesjms/36.2.158pmid: N/A
Fertilized eggs of seven species of the family Pleuronectidae were classified into two groups based on the presence or absence of an antigen (A) detected by both immunodiffusion and immunofluorescence. The immune sera used were produced by rabbits in response to intraperitoneal injections of homogenates of whole egg extracts and Freund's Complete Adjuvant. All species tested possessed a common antigen (B) whereas only four of the seven species possessed an additional antigen (A). The antisera produced with fresh and frozen eggs did not detectably react with chemically preserved eggs in either immunofluorescent or immunodiffusion test. Temperature of incubation was an important factor for the immunofluorescent method; fish eggs incubated at 37°C fluoresced with variable intensities, while at 0°C all eggs fluoresced with equal intensity.
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