Are we designing fishways for diversity? Potential selection on alternative phenotypes resulting from differential passage in brown trout

Fishways are commonly employed to improve river connectivity for fishes, but the extent to which they cater for natural phenotypic diversity has been insufficiently addressed. We measured differential upstream passage success of three wild brown trout (Salmo trutta) phenotypes (anadromous, freshwater-resident adult and parr-marked), encompassing a range of sizes and both sexes, at a Larinier superactive baffle fishway adjacent to a flow-gauging weir, using PIT telemetry (n = 160) and radio telemetry (n = 53, double tagged with PIT tags). Fish were captured and tagged downstream of the weir in the autumn pre-spawning period, 2017, in a tributary of the River Wear, England, where over 95% of tributary spawning habitat was available upstream of the weir. Of 57 trout that approached the weir-fishway complex, freshwater-resident adult and parr-marked phenotypes were less successful in passing than anadromous trout (25%, 36%, and 63% passage efficiency, respectively). Seventy-one percent of anadromous trout that passed upstream traversed the weir directly. Although the fishway facilitated upstream passage, it was poor in attracting fish of all phenotypes (overall attraction efficiency, 22.8%). A higher proportion (68.2%) of parr-marked trout that approached the weir were male and included sexually mature individuals, compared with that of freshwater-resident (37.8%) and anadromous trout (37.0%). The greater passage success of anadromous trout was likely due to their greater size and locomotory performance compared to the other phenotypes. Barriers and fishways can act as selection filters, likely the case in this study, and greater consideration needs to be given to supporting natural diversity in populations when proposing fishway designs to mitigate river connectivity problems.