Interacting effects of density and temperature on fish growth rates in freshwater protected populations

Abstract

Despite the demonstrated benefits of marine protected areas, there has been relatively little dialogue about freshwater protected areas (FPAs) even though some have been established to protect freshwater species from recreational and commercial fishers. After populations recover from fishing pressure, abundances and densities of formerly fished species increase, and we should therefore expect changes in demographic traits compared to those in exploited populations. To test this, we used capture-mark-recapture data for 10 Galaxias maculatus populations across a density gradient mediated by different degrees of fishery closure. We examined the extent to which density-dependent (DD) and density-independent (DI) effects interact to affect specific growth rates in post-recruit populations. We found that population density, stream temperature and individual size interact to affect growth rates. When population densities were high, compensatory responses of far slower growth rates were strongest, indicating that DD growth is a key mechanism regulating post-recruit populations of G. maculatus . This study emphasizes the importance of understanding DD and DI processes, their interactions, function and effectiveness for freshwater fisheries management. For FPAs to be effective, the extent and quality of target species' habitats must serve as key criteria for protection to alleviate competition for limited resources that underpins DD processes.

Protection from fishing improves body growth of an exploited species

“…Contrary to our findings, theoretical work on MPAs has assumed reduced body growth in MPAs due to density dependence, and therefore reduced fisheries yields outside MPAs due to spill-over of slow-growing (and smaller) individuals [35,61]. Although it is well documented that individual growth can be affected by compensatory density dependence in exploited populations [62][63][64][65], the effects of protection on body growth of aquatic animals have been shown to both be positive [33,[66][67][68][69] (also this study) and negative [34,36,70,71]. These mixed results illustrate that it is difficult to generalize how protection can affect body growth, because the outcome may depend on the interactions of many factors, such as the life histories of the species in question, the selectivity and intensity of the fishery, and the age, size and location of the reserve.…”

Closing the life‐history loop: Density effects on fecundity and egg size of an exploited, amphidromous fish (Galaxias maculatus) in freshwater protected areas

“…However, G. maculatus decreased in size with increased abundance. In a subsequent study, we found evidence of interacting effects of population density and water temperature on individual growth rates of post‐recruit G. maculatus (Watson et al, 2022). Specifically, increased abundance but reduced growth rates were observed in post‐recruit G. maculatus in fully protected (closed) areas.…”

“…Sites were classified as: (1) closed ( n = 3), where whitebait fishing is prohibited throughout the catchment; (2) partially closed ( n = 3), where whitebait fishing is not allowed in a tributary, but is permitted in the mainstem below the tributary; and (3) open ( n = 4), whitebait fishing is allowed throughout the catchment. The different degrees of fishery closure mediated a density gradient, with closed streams having the greatest G. maculatus population densities (see Watson et al, 2021 for more details), and far slower growth rates compared to low density (partially closed and open) streams (Watson et al, 2022).…”

“…To examine the role of density‐dependent and density‐independent processes, we followed a similar approach to Huntsman et al (2021) and Watson et al (2022) constructing linear mixed effects models capable of explaining variation in fish fecundity and egg size, and comparing them using Akaike's information criterion (AIC). First, to identify the body size metric that had the most substantial support for describing variation in fecundity and egg size, single‐variable linear mixed effects models of metrics for different body sizes were built (sensu Koops et al, 2004).…”

“…Conversely, Johnston (2018) found that faster‐growing female lake trout ( Salvelinus namaycush ) tended to produce larger eggs. Density‐dependent growth links the size of individuals to population density, and because smaller fish produce fewer eggs, the effect can propagate back to the population level (Watson et al, 2022). Therefore, understanding the consequences of density‐dependent growth on G. maculatus fecundity and egg size is vital for determining the potential of no‐take FPAs to enhance freshwater fisheries.…”