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

Abstract

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.

Fish morphology and passage through velocity barriers. Experience with northern straight-mouth nase (Pseudochondrostoma duriense Coelho, 1985) in an open channel flume

“…Moreover, it is estimated that more than 1.2 million obstacles fragment European rivers, most of them low head barriers (\ 2 m in height; Belletti et al, 2020). These structures hinder or block the movement of aquatic fauna, especially fish, which is a vital element of their life history (Strayer & Dudgeon, 2010;Lothian et al, 2020).…”

“…The selection pressure on fish populations and the characterization of the evolutionary impacts of velocity barriers have barely been tackled from a phenotypic perspective and never by quantifying morphological characters. In fact, most of them have been about salmonids and take into account features like size using traditional morphometrics (Marcus, 1990), but not morphological traits by removing size effect (Maynard et al, 2017;Lothian et al, 2020). Knowing the relationship between shape (sensu Benson, 1975) and swimming capacity of fish is essential to understand the selective pressure that velocity barriers in rivers and streams can exert on these organisms.…”

Investigating the phenology of juvenile potamodromous brown trout (Salmo trutta L.) in two large lake catchments

“…The presence of parr‐markings was used as a phenotypic indicator of juvenile state, and it was assumed that all tagged fish were immature. It is possible that a small proportion of the tagged samples, particularly males, could have matured in the autumn following tagging (Forty et al ., 2016; Lothian et al ., 2020). After each pass the electric fishing team identified and counted all fish before L F (mm) and weight (g) were measured, with the additional biological data from the tagging sub‐sample collated and added later.…”

“…It is possible that a small proportion of the tagged samples, particularly males, could have matured in the autumn following tagging (Forty et al, 2016;Lothian et al, 2020). After each pass the electric fishing team identified and counted all fish before L F (mm) and weight Inserts show the distribution of receiver arrays in each study catchment (Zippin, 1958).…”

The role of individual behavioral traits on fishway passage attempt behavior

“…In northern temperate zones, river fragmentation has heavily impacted on brown trout populations. This is largely a result of size‐related passage ability and selectivity by barriers to migration whereby larger individuals are more likely to succeed in passing upstream of an obstacle (Jones et al., 2021; Lothian et al., 2020; Noonan et al., 2012). Given that the reproductive component of a population can stem from any of the phenotypes which can be classified into discrete size groupings (Birnie‐Gauvin et al., 2019; Ferguson et al., 2019), smaller phenotypes can be severely curtailed in their migratory range, potentially being denied access to suitable spawning grounds.…”