DOI: 10.5061/dryad.mcvdnck5c
title: Do highly anthropized hydrological conditions in marshes influence fish communities according to their life‐history strategies?

description: AbstractAlterations of natural hydrology in aquatic ecosystems are known
 to strongly impact the community composition of different taxa.
 Surprisingly, literature on the potential influence of hydrology on fish
 community composition is still very scarce in agricultural marshes, where
 canals represent one of the few remaining aquatic habitats. This study is
 aimed to address this research gap by monitoring fish communities in
 independent hydrological units differing in hydrology management over a
 6 years period. We predicted variable fish responses to the hydrological
 context according to different life‐history strategies (opportunistic,
 equilibrium, or periodic species). Periodic and opportunistic species were
 the most frequently observed. Despite differences in hydrology between
 canals (but little variation over years), we found that hydrology
 explained only a very low proportion of variation in the composition of
 fish communities. In particular, the flooding duration of meadows in early
 spring did not influence the composition of fish communities, not even the
 abundance of periodic species expected to rely on such temporary habitats.
 Instead, fish communities were more influenced by local habitat variables
 (aquatic vegetation cover, turbidity, tree roots, and refuges under the
 canal banks). The hydrological management of most hydrological units for
 agricultural purposes (i.e., severe flood abatement in spring and shallow
 water depth in canals in summer) was found to be incompatible with
 conservation goals to promote more diverse fish communities between
 hydrological units. Therefore, we call for further investigations in
 similar habitats covering a larger range of hydrological conditions.
description: The study was conducted in the
 Marais poitevin, a ca. 1000-km<sup>2</sup> French hypertrophic
 marshland located along the Atlantic coast. It has been historically
 divided into different hydrological units in which water management is
 controlled independently from the surrounding units because of different
 water regulations fixed by management committees. This leads to different
 hydrological patterns across units. We selected eleven hydrological units
 to represent the diversity of hydrology conditions in the study
 area.<br> Fish sampling<br>
 Each year, fish were sampled
 between the last week of June and the second week of July.
 Fish were sampled using
 the point abundance sampling approach (PAS; Copp and Persat, 1989; Nelva et al.,
 1979) with an
 electrofishing apparatus (Heron®, Aigrette®, or EFKO F.E.G. 8000® depending on the year,
 voltage during fishing operations 255±75V, amperage 7±5A). A total of 30 PAS were performed per canal,
 spaced out by at least 10 meters to provide independent samples. At each
 PAS, the anode was immersed about five meters in front of the boat and
 near the canal bank where fish generally concentrate.
 All shocked fish within 1
 m<sup>2</sup> of the impact point of the anode were caught,
 identified to the species level, measured (fork length, mm), and released
 back into the water. Environmental variables
 The water depth was calculated
 for different periods, and measured at two different spatial scales:
 locally, in the canal where fish were sampled, and across 10 small nearby
 canals connected to the fish canal in order to better characterize the
 hydrological conditions of the hydrological unit (average value).
 We calculated the average water depth for two seasons: winter
 and spring. Two additional hydrological variables were calculated at the
 hydrological unit scale to describe the rate of water depth changes at the
 pivotal period in early spring, namely the coefficients of variation of
 water depth in March and in April. We also assessed the amount of
 temporary fish-accessible aquatic habitats as expressed by the flooding
 duration (days) of at least 5% and at least 20% of the meadows in the
 whole hydrological unit. Calculations were done for spring and winter so
 that four flooding duration variables (two quantiles × two seasons) were
 used for subsequent analyses.<br><br>We also considered a
 series of water physico-chemistry variables, most of them collected as
 part of a companion monitoring program on the eutrophication of the Marais
 poitevin. Chlorophyll a content (µg.L<sup>-1</sup>), oxygen
 percentage saturation (%), conductivity (µS.cm<sup>-1</sup>),
 and ammonium, nitrite, nitrate and orthophosphate concentrations
 (mg.L<sup>-1</sup>) were measured in one large canal
 representative of each hydrological unit, all year round. We also measured
 basic variables – water temperature (°C), conductivity
 (µS.cm<sup>-1</sup>) and turbidity (m) – in the fish canal
 once a year, when sampling the fish. Lastly, we measured five habitat
 features: aquatic vegetation cover (%), tree roots and refuges under the
 banks, width (m) of the canal section where fish were sampled, and the
 total network length (km/ha) of large permanent canals and that of
 narrower temporarily dried canals in the hydrological unit.
 Please read the article
 entitled "<em>Do highly anthropized hydrological
 conditions in marshes influence fish communities according to their
 life-history strategies?</em>" in River Research and
 Applications for more details.<br>