Why, when and how do fish populations decline, collapse and recover? The example of brown trout (Salmo trutta) in Rio Chaballos (northwestern Spain)

Abstract

Why, when and how do fish populations decline, collapse and recover? The example of brown trout (Salmo trutta) in Rio Chaballos (northwestern Spain) JAVIER LOBÓ N-CERVIÁ Museo Nacional de Ciencias Naturales (CSIC), C ⁄ José Gutiérrez Abascal, Madrid, Spain SUMMARY 1. Around the year 2000, historically abundant populations of brown trout (Salmo trutta) and Atlantic salmon (S. salar) co-occurring in rivers flowing along the Cantabrian corridor of north-western Spain showed a dramatic decline to alarming levels. For one reason or another, fishing was not banned and fishing pressure continued to reduce the few survivors. Unexpectedly, the populations recovered 'naturally' in a very short time period to the extent that in 2004-06 numbers had attained population sizes comparable to those of the mid 1980s. 2. The population of brown trout in Rio Chaballos showed a boom-and-bust pattern concurrent with those observed across broader geographical scales. This study revisits a 22-year data set to explore the nature of the severe decline and rapid recovery of this population. 3. Recruitment was related to stream discharge in March that covers the emergence period and the earliest search stages for food. Coefficients of variation for discharge and recruitment increased over the years and were highly correlated with each other, demonstrating that increased temporal variability in recruitment is strongly linked to increased variability in stream discharge. In turn, recruitment appears to be the major determinant of year-class strength and hence, of population size. 4. A number of factors appeared to operate as resilience mechanisms, permitting the population to increase rapidly when environmental conditions are optimal. These include strong recruitment-discharge relationships, short life-span, small stocks of eggs or offspring required to fill the amount of space suitable for the youngest juveniles and a few females surviving to spawn for a second time. 5. Implications for fishery management and conservation are discussed in the context of the expectation that these populations will vary over the coming years within ranges similar to those in this study.

Sibship reconstruction for inferring mating systems, dispersal and effective population size in headwater brook trout (Salvelinus fontinalis) populations

“…Variance in reproductive success and unequal sex ratio reduce N e in wild populations (Frankham 1995), but there was no evidence that these factors were operating in our study. The N e value is also reduced when a population size fluctuates over time (Frankham 1995); stream salmonids are known to exhibit such temporal patterns (Elliott 1994;Lobón-Cerviá 2009), although data is not temporally available in our study sites. Headwater habitat is inherently small with limited physical space.…”

A Long-Term Study of Ecological Impacts of River Channelization on the Population of an Endangered Fish: Lessons Learned for Assessment and Restoration

“…Further, because population responses integrate environmental effects on behavior, growth, survival, and reproduction of many individuals, timelines for population responses may include time lags and other forms of temporal variation. For example, fish populations may exhibit region-wide boom-bust patterns that reflect annual variation in recruitment [30]. High inter-annual variation in fish abundance can preclude detection of impacts and may not provide feedback sensitive or rapid enough to prevent impacts from causing unacceptable harm, especially for rare taxa [31].…”

The influence of natural variation in discharge on juvenile brown trout population dynamics in a nursery tributary of the Motueka River, New Zealand

“…Floods can have severe adverse effects at all stages of the life cycle but usually during late incubation and emergence of fry (Allen 1951;Elliott 1994;Jowett & Richardson 1989;Hayes 1995;Lobo´n-Cervia´1996;Harvey et al 1999;Jensen & Johnsen 1999;Nislow et al 2002), significantly affecting year-class strength (Mortensen 1982;Elliott 1987;Nehring and Anderson 1993;Cattaneo et al 2002;Lobo´n-Cervia´2009). In addition to direct mortality caused by floods, associated high water velocity may also displace fry (Ottaway & Forrest 1983;Crisp & Hurley 1991) and reduce their suitable habitat (Nehring & Anderson 1993) and food availability (Elwood & Waters 1969).…”

“…Wondzell & Swanson 1999). In many lotic systems, flooding is the primary source of disturbance and may affect community structure, and particularly fish populations, long after the floodwaters have receded (Resh et al 1988;Biggs & Close 1989;Matthaei et al 2003;Olsen et al 2007;Effenberger et al 2008;Lobo´n-Cervia´2009). Such effects can result from differences in the resistance or resilience between species, or as a result of habitat modification caused by the disturbance.…”

“…The relationship between density and body weight is used for assessing population selfthinning and has proved to be a useful index of density-dependent loss (Elliott 1993a;Lobo´n-Cervia´2008). However, floods and drought have a much greater influence in controlling salmonid populations (Jowett & Richardson 1989;Harvey et al 1999;Nislow et al 2002), the former largely through recruitment and especially so in hydrologically variable streams (Hayes 1995;Lobo´n-Cervia´2009;Nicola et al 2009). Therefore, the highly variable flow regimes common throughout New Zealand (Jowett & Duncan 1990) ought to be the dominant influence on salmonid and native fish populations.…”