Resource limitation during early ontogeny: constraints induced by growth capacity in larval and juvenile fish

The presence of and mechanisms behind density-dependent growth and resource limitation in larval and juvenile stages of organisms with high mortality such as fish are much debated. We compare observed consumption and growth r ates with maximum consumption and growth rates to study the extent of resou rce limitation in young-of-the-year (YOY) roach (Rutilus rutilus) and perch (Perca fluviatilis). Diet, habitat use, consumption rate and growth rate w ere measured under varying YOY fish densities over 2 years in four lakes. I n the first year, YOY roach and perch were studied under allopatric conditi ons. Experimental addition of perch roe in the second year also allowed stu dy of YOY of the two species under sympatric conditions in two of the lakes . The diet of YOY roach was dominated by cladoceran zooplankton and YOY roa ch habitat use was restricted to the shore region in both years. This restr icted habitat use did not involve any cost in foraging gain in the first ye ar as consumption and growth rates were very close to maximum rates. During the second year, when the two species coexisted, resources were limited in late season, more so in the littoral than in the pelagic habitat in one la ke while the reverse was the case in the other lake. The diet of YOY perch was also dominated by zooplankton, and with increasing perch size the propo rtion of macroinvertebrate prey in the diet increased. After hatching, YOY perch first utilized the pelagic habitat restricting their habitat use to t he shore after 1 to several weeks in the pelagic zone. During the larval pe riod, perch were not resource limited whereas juvenile perch were resource limited in both years. The fact that YOY perch were more resource limited t han YOY roach was related to the higher handling capacity and lower attack rate of perch relative to roach, rendering perch more prone to resource lim itation. Estimates of resource limitation based on consumption rates and gr owth rates yielded similar results. This supports the adequacy of our appro ach to measure resource limitation and suggests that this method is useful for studying resource limitation in organisms with indeterminate growth. Ou r results support the view that density-dependent growth is rare in larval stages. We suggest that density-dependent growth was absent because larval perch and roach were feeding at maximum levels over a wide range of larvae densities.