Variation In Year-class Strength Research Articles

Some Factors Associated with Fluctuation in Year-class Strength of Sauger, Lewis and Clark Lake, South Dakota

Abstract The strength of sauger, Stizostedion canadense (Smith), year classes in Lewis and Clark Lake has fluctuated widely since formation of the reservoir in 1956. Our studies suggest that more than 80% of the variability in year-class strength measured from the catch of fish older than age 0 can be predicted from knowledge of water level change over the spawning ground, June reservoir water temperature, and reservoir water exchange rate. These factors were measured and probably function during the first 2 months of fish life. Sauger year classes of better than average strength can be expected in most years if both power-peaking operations at Fort Randall Dam during the fish spawning and incubation period, and the reservoir water exchange rate during June are minimized.

Derzhavin's Biostatistical Method of Population Analysis

Derzhavin's biostatistical method, as modified by Boiko and others, uses annual age censuses and catch statistics to compute the utilized stock (virtual population), Vi, of year-class i in a given year as the sum of all members of that year-class that are caught in the current and future years. The sum of Vi values for all year-classes present in a given year is the total utilized population, V. The biostatistical rate of exploitation for individual ages (Ci/Vi), or for the stock as a whole (C/V), is the catch divided by the utilized stock. If ages of the fish in the stock are read too low, values of Ci/Vi and C/V are too large. For fully recruited fish, Ci/Vi is an unbiased estimate of the total mortality rate, A, bat for incompletely recruited ages, it differs progressively from A and has no simple interpretation. Variations in abundance of successive year-classes do not affect the interpretation of Ci/Vi or C/V for fully recruited ages, but changes in rate of fishing, F, are disturbing. When incompletely recruited ages are included in a biostatistical computation (as has usually been done), C/V does not have any precise interpretation in terms of population parameters. In addition, variations in year-class strength affect the value of C/V even when F does not change from year to year. It is possible to use C/V for the whole catch as an "arbitrary" index that reflects the direction and magnitude of changes in intensity of fishing over a period of years. However, even for this purpose the same minimum age must be used in all years. When data for a long series of years are available, the utilized year-class strength (ΣCi) of the year-class hatched in year i can be compared with the total utilized stock (or the utilized mature stock) present in year i, in order to examine parent–progeny relations. Incompletely recruited as well as fully recruited ages can be used in this comparison, but adjustments are necessary if mortality rate (natural or fishing) changes from year to year. The biostatistical method does not provide separate estimates of natural and fishing mortality rates. If the possibility of a moderate or large natural mortality is disregarded, serious errors of interpretation may result.

Effects of Artificial Propagation and the Weather on Recruitment in the Lake Ontario Whitefish Fishery

No significant relationship could be found between variations in the level of fry planting and ensuing variations in the level of catch in the long-term statistics of the Lake Ontario whitefish gill-net fishery. An alternate-year planting experiment in which fry were planted in the even-numbered years 1944 through 1954, similarly failed to show a detectable level of contribution of the hatchery fish to the fishery. The supported year-classes averaged larger than those not given hatchery support but this was judged coincidental because of a phasing with an alternate-year periodicity which characterized the catch statistics over the whole series examined, and was present in the six years prior to the start of the alternate-year plantings. Estimates of spawning stock were calculated and it was not found that the progeny-per-parent ratios were significantly affected by the fry plantings.A possible explanation for the periodicity was suggested by a significant correlation between air temperatures at the times of spawning and hatching, and the strengths of the produced year-classes. Cold Novembers followed by warm Aprils appeared to provide conditions associated with the production of larger year-classes, with the opposite combinations relating to the weaker broods.The disappearance of lake trout and ciscoes from the commercial catch resulted in increased fishing pressure directed towards the capture of the whitefish. Increases in the efficiency and intensity of the fishing were observed during the period of the study and these were likely responsible for the reduction of the average age of the fish in the catch by almost one year, and the resultant restriction of the annual catch to one year-class in recent years. Probably because most of the fish are currently caught before first spawning, a decline in spawning stock was almost continuous during the study period. The year-to-year fluctuations in the level of the catch increased both because of the reduced average age, and because of a greater variation in year-class strength. The year-classes produced in favourable years tended to increase, down to quite low levels of stock. This gave a configuration similar to Ricker's Type C reproduction curve, and which differed chiefly in that the limiting diagonal representing minimum reproduction fell below the replacement diagonal. It was suggested that the whitefish may require more than one spawning, to achieve stock replacement. The present instability of the catch, and the potentially serious effect of any sustained break in the rhythm of the climatic conditions suggest a condition of over-exploitation in this case.

The Smallmouth Bass Fishery of South Bay, Lake Huron

The smallmouth bass (Micropterus dolomieui) population of South Bay, Lake Huron and the sport fishery imposed on it have been studied by means of marking and creel census carried out during the years 1947 to 1952 inclusive. Population estimates of legal-sized bass (over 10 inches–fork length), based on the recovery of marked fish, show a decrease from 5,900 fish in 1947 to 2,900 in 1949 with an increase again to 5,700 in 1952. The number of bass caught per 100 rod-hours in these years was 89, 51 and 87, respectively. The recovery of the population from its low in 1949 was due solely to the 1947 year-class which dominated the fishery in 1950, 1951 and 1952, contributing 34%, 71% and 67%, respectively, of the anglers' catch in these years. Analysis of scale samples of the anglers' catch shows considerable variation in year-class strength. In contrast to the 3,500 fish contributed to the anglers' creel by the 1947 year-class, the 1945 year-class contributed only 400 fish.South Bay bass enter the sport fishery in small numbers at age III and 70% of the anglers' catch consisted of age-III to age-V fish. Sixty per cent of the legal-sized population of bass present in South Bay in one year are taken by anglers in the succeeding 4–5 years at the present exploitation.A large percentage of the bass population is located in the Inner Basin of South Bay where they are dispersed in varying concentrations along the shoreline. During the five-year study 3,331 bass were tagged and, although the loss of tags was high, 692 were recaptured by trap nets in the same season they were released. Eighty-five per cent of these were taken within two miles of their release point. Of 60 bass recaptured a year after tagging, 43 (72%) were taken within two miles of their release point. Only five per cent of all recorded recaptures were caught over five miles from their release point. Larger bass tended to range farther than small bass.

Growth, Maturity and Mortality in the Relatively Unexploited Lake Trout, Cristivomer namaycush, of Great Slave Lake

Ages of 9,215 lake trout were determined. Length-weight and time-maturity relationships are shown. Sex ratio is 1:1. Only half the females of mature size spawn each year. There is no growth except during June to September, inclusive. There is a tremendous difference in size at a given age. Growth is much faster in one half of the lake than in the other, it is the same as or slightly slower than in lakes farther south and considerably faster than in Great Bear Lake. Seven years of commercial fishing did not increase total annual mortality by much. Mortality rate increases with age among all fish of catchable size. A given year-class probably attains maximum biomass well before most of its members reach commercial size. No significant variation in year-class strength was found.

A Study of the Lake Trout, Salvelinus Namaycush, in two Algonquin Park, Ontario, Lakes

A study was made of the depth distribution, feeding, and growth of the lake trout, Salvelinus namaycush, in Lake Louisa and Redrock Lake, Ontario, in 1947. In the spring the trout were widely dispersed in Redrock Lake but as surface waters warmed to 14° or 15° C. in mid-June they moved into deeper waters. During the summer months the Redrock lake trout concentrated in the lower thermocline and upper hypolimnion while those in Lake Louisa were more widespread. Both populations frequently penetrated the 15- to 20-foot zone in mid-summer up to the 18° C. isotherm. In each lake the smaller trout had a deeper distribution. In both Redrock and Louisa the food was varied in the spring months. Insects, particularly the immature stages, were eaten extensively at this time. During the summer the Louisa trout fed on plankton as there was no other available food. The Redrock trout fed almost exclusively on the yellow perch, Perca flavescens, the distribution of these two species overlapping in the 12° to 18° C. zone. In both lakes a large number of empty stomachs occurred in the early fall. Small trout ate more plankton and insects and less fish than did the larger trout. Food competition and cannibalism were of little importance. Lake Louisa trout grow more slowly than those in Redrock Lake. This difference is correlated with the difference in feeding habits. The Lake Louisa fish have a growth rate similar to other plankton-feeding populations in Algonquin Park while the Redrock trout have growth like that of piscivorous populations. The lake trout of both lakes grow slowly as compared with many growth rates reported for the species. Maturity is reached at age V in both lakes. Variation in year class strength is evident in both Redrock and Louisa lakes.