Timing is everything: Fishing‐season placement may represent the most important angling‐induced evolutionary pressure on Atlantic salmon populations

Abstract

Fisheries‐induced evolution can change the trajectory of wild fish populations by selectively targeting certain phenotypes. For important fish species like Atlantic salmon, this could have large implications for their conservation and management. Most salmon rivers are managed by specifying an angling season of predetermined length based on population demography, which is typically established from catch statistics. Given the circularity of using catch statistics to estimate demographic parameters, it may be difficult to quantify the selective nature of angling and its evolutionary impact. In the River Etne in Norway, a recently installed trap permits daily sampling of fish entering the river, some of which are subsequently captured by anglers upstream. Here, we used 31 microsatellites to establish an individual DNA profile for salmon entering the trap, and for many of those subsequently captured by anglers. These data permitted us to investigate time of rod capture relative to river entry, potential body size‐selective harvest, and environmental variables associated with river entry. Larger, older fish entered the river earlier than smaller, younger fish of both sexes, and larger, older females were more abundant than males and vice versa. There was good agreement between the sizes of fish harvested by angling, and the size distribution of the population sampled on the trap. These results demonstrate that at least in this river, and with the current timing of the season, the angling catch reflects the population's demographics and there is no evidence of size‐selective harvest. We also demonstrated that the probability of being caught by angling declines quickly after river entry. Collectively, these data indicate that that the timing of the fishing season, in relation to the upstream migration patterns of the different demographics of the population, likely represents the most significant directional evolutionary force imposed by angling.

An extensive common‐garden study with domesticated and wild Atlantic salmon in the wild reveals impact on smolt production and shifts in fitness traits

“…Those studies found that the wild type dominated completely among salmon that returned as 1SW fish while among 2SW fish, hybrids and domesticated type dominated. In the salmon population inhabiting the river Etne however, the majority of salmon returned after 2 years in the sea (Harvey, Tang, Wennevik, Skaala, & Glover, ). As a result, we observed that the wild type tended to be older than hybrids and domesticated types at return to spawn, albeit with sex differences.…”

Swimming with the fishes: validating drift diving to identify farmed Atlantic salmon escapees in the wild

“…adequate water discharge). This could result in a mismatch be tween sampling and the effective spawning population (Harvey et al 2017). It has also been shown that the angling catch efficiency is quickly reduced after river entry (Harvey et al 2017), suggesting that variation in time of river entry (Erkinaro et al 2010, Svenning et al 2017) may cause different catch probabilities between wild and escaped salmon.…”

“…This could result in a mismatch be tween sampling and the effective spawning population (Harvey et al 2017). It has also been shown that the angling catch efficiency is quickly reduced after river entry (Harvey et al 2017), suggesting that variation in time of river entry (Erkinaro et al 2010, Svenning et al 2017) may cause different catch probabilities between wild and escaped salmon. Furthermore, differences within the river in the distribution of escaped and wild salmon and variations in spatial CPUE during fishing may further underestimate or overestimate the proportion of escaped farmed salmon in the spawning populations (Moe et al 2016).…”

Cryptic introgression: evidence that selection and plasticity mask the full phenotypic potential of domesticated Atlantic salmon in the wild

“…In the few cases where individuals shared an identical genetic profile to another fish, they were genotyped for a further five microsatellites to provide unambiguous individual identification. For full details regarding this procedure the reader is referred to previous work 59 , 60 .…”