Water abstraction affects abundance, size-structure and growth of two threatened cyprinid fishes

Abstract

Hydrologic alteration is a major threat to freshwater biota, and particularly fish, in many river courses around the world. We analyzed and compared the effects of water abstraction on two threatened cyprinid fishes of contrasting ecology (the Mediterranean barbel Barbus meridionalis and the Catalan chub Squalius laietanus) in a Mediterranean stream. We compared abundance, size-structure, growth, and condition of both species across perennial and artificially intermittent reaches affected by water abstraction. Both species were less abundant, had scarce large individuals, and displayed slower growth rates (length-at-age) in intermittent reaches, showing clear detrimental effects of water diversion. Mixed-effect models of scale increments showed variation among individuals and among sites, years and age classes for both species. The larger-sized, water-column species (chub) disappeared or was rare in many intermittent reaches. The barbel present in intermittent reaches showed better somatic condition than in sites with permanent flow, perhaps due to reduced competition after rewetting or colonization by better fitted individuals. This benthic, rheophilic species seems more resilient to moderate water abstraction than chub. Many effects of water flow intermittency were only detected on fish life-history traits when accounting for natural, often non-linear, variation, along upstream-downstream gradients. Our results suggest that abundance was the strongest indicator of effects of water abstraction on fish populations, whereas condition was a more labile trait, rapidly recovering from anthropogenic disturbance.

Fig 1. Location of the study area: Tordera Stream (T1‒T12) and its tributary, Arbúcies Stream (A1‒A3).

The flow regime during the study period is also shown. See Table 1 for further details on the sampling sites.

Fig 2. Relationship of mean fish abundance (May 2012‒October 2013) with altitude by flow regime and fish species.

The lines correspond to linear (S. laietanus, top plot) or quadratic regressions (B. meridionalis, bottom plot) separated by flow regime. Linear r² are 0.668 (perennial) and 0.620 (intermittent). Quadratic $R_{adj}^2$ are 0.407 (perennial) and 0.223 (intermittent). Sites T1‒T12 are located on Tordera mainstem; sites A1‒A3 are on the Arbúcies tributary. Squalius laietanus was not captured at sites T3, T5, T6, A1, and A2.

Fig 3. Maximum and mean size of fish captured in the Tordera basin during 2012 and 2013, comparing perennial and intermittent reaches.

Boxes represent the first and third quartiles, lines are the medians, and bars are maximum and minimum values, excluding outliers (circles).

Fig 4. Relationship of fish condition (total weight adjusted for fork length with analysis of covariance) of B. meridionalis with altitude.

The lines correspond to quadratic regressions by water flow regime. Total weight and fork length were log-transformed. Quadratic $R_{adj}^2$ was 0.406 for perennial reaches and 0.375 for the intermittent reaches.

Fig 5. Marginal means of fish condition (total weight adjusted for fork length with analysis of covariance) across sampling sites (Table 2).

The mean log₁₀ of fork length (at which fish total weight was adjusted) was 2.165 for chub and 2.01 for barbel. Non-estimable means are not shown. Sites T1‒T11 are located on Tordera mainstem and sites A1‒A3 are on the Arbúcies tributary.

Fig 6. Marginal means of fish fork length (adjusted for fish age with analysis of covariance) across sampling sites (Table 2, below).

The mean age (at which fish size was adjusted) was 2.82 years for S. laietanus and 2.83 years for B. meridionalis. Non-estimable means are not shown. Sites T1‒T11 are located on Tordera mainstem; sites A1‒A3 are on the Arbúcies tributary.

Fig 7. Effects of fish age, sampling site and year of growth on yearly increments of fish scale oral radius, as estimated by the mixed-effects models.

See Table 3 and text for significance of the factors. The bars how standard errors. Codes of sampling sites as in Table 1.