Variation In Recruitment Rates Research Articles

Spatial and temporal variation in recruitment and its effects on regulation of parasite populations

Variation in recruitment rates of parasites to hosts possibly contributes significantly to fluctuations in parasite numbers, yet is almost never measured directly in the field. I measured the variation in recruitment rates of three species of parasitic mites living in two species of freshwater mussels over several spatial and temporal scales. I also examined separately the effect of spatial dispersion of hosts on mite recruitment. Uninfected hosts of both species were placed out each month, for a period of a month, for 2 years at one site and 12 months at another. Mussels of both species were collected simultaneously each month so that abundance of recruiting mites could be compared to mean abundances of mites in hosts at that time. To test the effect of host dispersion on recruitment rates, mussels were set out in clumped and regular patterns in a separate experiment. Overall, recruitment rates were often high but also varied substantially between sites, seasons, years and months. The likely impacts differed between mite species with one probably affected strongly by recruitment variation, while abundances of the other two were not. Populations of the latter two species are probably regulated by intraspecific competition for mates and egg-laying sites. Sampling data are often used to estimate recruitment rates but the latter should be measured, if possible, by exposing uninfected hosts for a known period of time. This direct method reveals patterns of recruitment that cannot be deduced from sampling data. The lack of information on recruitment variation represents a major gap in our knowledge of parasite populations.

Monitoring fish stocks in relation to acidification in Norwegian watersheds

The Norwegian Monitoring Programme for Long-Range Transported Air Pollutants started in 1980. The biological part of this programme includes besides invertebrate studies in streams, (i) fish community status in lakes by means of interviews, test-fishing in lakes by using standard gill-net series, recruitment studies of brown trout in inland streams, and juvenile stock assess and monitoring of fish kills in salmon rivers. Damaged fish stocks are recognized within a land area of 51,500 km2 in southern Norway and 30 km2 in northern Norway. At least 6,000 lake-dwelling fish stocks have either been lost or are at various stages of reduction. Brown trout (Salmo trutta) is the most widespread and abundant species of fish in Norwegian watersheds, and is also most severe affected by acidification. More recently, there are some indications of an increase in the abundance of brown trout in some areas. However, analysis of age structure in lakes, and fry densities in streams in such areas revealed large annual variations in recruitment rate, which indicates unstable water chemical conditions. Atlantic salmon (Salmo salar) is virtually extinct in 25 rivers in southernmost, southwestern and western Norway.

A microcomputer program for stimulating effects of physical transport processes on fish larvae

ABSTRACTA simple microcomputer program uses Lagrangian particle tracking to simulate the fates of individual larvae subject to wind‐ and tide‐driven advection fields and diffusion. Space/time advection patterns are provided to the program as inputs; thus, the program can use results from various hydrodynamic models. The program can simultaneously track several groups of larvae that are assigned varying attributes, including (1) body size, to allow simulation of effects of variation in growth rates; (2) spawning locations and times; (3) vertical migration behavior; and (4) settlement strategy (duration of larval period, depth conditions for settlement). The model is intended as an exploratory tool to help identify alternative hypotheses that might explain observed life history patterns and causes of inter‐annual variation in recruitment rate. For English sole in the Hecate Strait, B.C., the model leads us to hypothesize that observed spawning locations have resulted from a tradeoff between places that would provide the best feeding opportunities and places that would minimize risk of advective export to unfavorable rearing habitats.