No abstract

Fluctuations in mortality rather than in growth rates produced most of the year-to-year differences in biomass accumulation of three different populations of the crayfish Orconectes virilis. Yearly biomass changes resulted from density-dependent control of mortality and fecundity during certain portions of the life cycle. Density-dependent changes in mortality rates controlled population size for adults in all lakes and for young-of-the-year in two of the three study lakes. Growth rates were much less responsive to fluctuating densities. Disparity between the number of ovarian and attached eggs increased as density of age I+ crayfish increased. This provided a strong density regulator on fecundity. Differences occurred in the number of recruits produced by a brood stock that survive to the end of the first growing season in the various lakes. Yet the number of females surviving to reproductive age 2 yr later was strongly regulated. Strong population regulation produced two female recruits of breeding age for every two–six parental breeding females. The high biomass and production levels of crayfish discovered in West Lost Lake in 1962–63 also occurred in the other area lakes. Higher but variable levels of recruitment resulted in larger standing crops and production in West Lost lake. This resulted from less effective density controls on the mortality rates of younger age-groups in that lake. Despite great variation in biomass of from 46 to 213 kg/ha and annual production from 60 to 142 kg/ha, the annual turnover ratio of the biomass was found to vary only between 0.94 and 1.53. Key words: crayfish, Orconectes virilis, population dynamics, annual production, Michigan lakes

Field study revealed great differences in the biological productivity of two adjacent areas of a Michigan trout stream resulting from the entrance of domestic sewage into the stream between the two areas. Monthly samples were collected from the two areas to determine the seasonal cycles in abundance of bottom fauna, feeding habits of the trout, and coefficient of condition of the brown trout.In the less productive area upstream, a paucity of food of aquatic origin caused a sharp decline in condition of the fish, a reduction in the quantity of food per stomach, and a shift to a diet containing a considerable portion of terrestrial organisms.In the more productive area downstream (which, throughout the year, had a greater volume of bottom fauna than the unproductive area) trout maintained a significantly higher and much less variable coefficient of condition, their stomachs contained more food in midsummer and did not show the increase in terrestrial foods.

A mathematical model of trout‐stream fisheries was developed that can be used to evaluate a variety of fishing regulations. Density‐dependent mortality was found in the first 2 years of life for each of the two brook trout (Salvelinus fontinalis) and three brown trout (Salmo trutta) populations studied in Michigan. Regression equations were used to describe the density‐dependent relationships for modeling purposes. Equations were developed that used mortality, growth, and length‐frequency information to calculate the number of fish in a population, number caught and harvested, number caught and released, number of deaths due to hooking mortality, number of natural deaths, and number recruited for any time period and age‐group. Also, addition of a length‐weight regression allowed equations to be developed for calculating yield in weight harvested, yield in weight caught and released, and gross biomass production for any time period and age‐group. Effects of imposing different types of length limits, including minimum, inverted, or slot limits, can be analyzed with this mathematical technique. Fishing mortality and hooking mortality can be adjusted to simulate values typical for different gear types. In addition, consequences of seasonal fluctuations in growth and fishing mortality, including shifts in length of fishing season or time frame, can be assessed. The equations were incorporated into a computer simulator, TROUT.DYNAMICS, and the brown trout fishery in a section of the Au Sable River, Michigan, was simulated for a period in the past and a period in the future to demonstrate applications of the model.

In an earlier study during 1962 and 1963, movement of female Orconectes virilis from shallow to deeper water in summer was noted in West Lost Lake, Otsego County, Michigan. The present additional observations on O. virilis, during three summers and in three lakes, confirm this migration. Evidence from other areas suggest that this movement is a general phenomenon of the species throughout most of its range. Migration of females appears to be associated essentially with muturation of the gonads.

Populations of the crayfish Orconectes virilis were exploited using the Paulik–Bayliff modification of the Ricker model in two small lakes. After harvesting with traps at maximum sustained yield (MSY) for 3 yr, fishing effort was increased fourfold in one lake, West Lost, for two successive seasons, while in North Twin Lake it was not changed. In West Lost yields were stable during MSY; yields increased with overexploitation, but growth and recruitment declined. Distortions occurred in the age composition of the catch. In North Twin this pattern was repeated except that in 1975 the catch dropped drastically even though fishing effort had not been increased. Because of declining recruitment, North Twin was actually overfished by about 20% in 1974 and greatly overfished in 1975. Decreased recruitment in both lakes can be attributed to two factors: (1) a decreased survival of hatchlings in both lakes due to a decline in nursery habitat; as a consequence the stock-recruitment relationship was altered; and (2) an increase in the density of age I (subadult) females due to the selective fishery for males; as a result density-dependent mechanisms lowered the survival of females from age I to II, producing fewer spawning adults. However, the high density of these age II females at age I decreased the egg production per adult female. The changes in the productivity of the lakes for crayfish during the period of exploitation were related to loss of microhabitat for hatchlings. This change in productivity offset the benefits of MSY management and resulted in a drastic drop in stocks when deliberate overfishing took place. West Lost gave good yields prior to its certain collapse. Yields in weight were an unreliable indicator of the impact of the fishery. Key words: crayfish, Orconectes virilis, potential yield, production, recruitment, stock assessment, experimental fishery

No abstract

The crayfish Orconectes virilis is a major component of the benthos of three small lakes in northern Michigan. These lakes contained stocked brook trout (Salvelinus fontinalis) populations (age-0 and age-I) at densities of 188, 411, and 1398 fish/ha. Crayfish were preyed upon by brook trout, but only during their first year of life. Two-year production of age-0 crayfish was approximately 94 kg in each lake; brook trout consumed only about 1–2% in lakes with lower density offish. Even with greater trout density and accompanying higher rate of exploitation (40%), there was no impact on recruitment of young crayfish because compensatory mortality of crayfish occurred in lakes with less trout predation. However, stocking trout at higher densities decreased trout growth and condition. While 2-yr production was highest (59 kg/ha) in the lake stocked at the highest density and lowest (17 kg/ha) in one stocked at the lowest density, the former was achieved at the cost of a very slow growing trout population. Most of the crayfish production is not utilized as trout food but enters the non-predatory pool of detrital organic matter. Key words: Orconectes virilis, brook trout, predation, production growth, mortality