A common garden experiment in the wild reveals heritable differences in migration tendencies among brown trout populations

Abstract

We undertook a common garden experiment in the Burrishoole catchment, western Ireland, to test for heritable life-history differences among neighboring brown trout (Salmo trutta L.) populations that exhibit neutral genetic divergence. Experimental crosses were made using either local females (obtained from a below-waterfalls section of the Rough River within the Burrishoole) or females from the Erriff River-a neighboring catchment that currently produces a stronger run of anadromous migrants than the Burrishoole. Each female was mated to three different types of males: Rough Below-Falls, Rough Above-Falls (resident males obtained from above the waterfalls), and Erriff. Offspring from the resulting six crosses were introduced as unfed fry into a stretch of the Rough River bounded upstream by the waterfalls and downstream by a Wolf-type fish trap (Rough River Downstream Trap, RRDT). Genetic parentage analysis (16 microsatellite markers) was then used to assign offspring sampled at various time points and locations back to cross type. No differences in parr survival rates (electrofishing in the Rough River) were found among the crosses, but parr moving downstream (intercepted at the RRDT) were skewed toward the Erriff female × Erriff male cross, with a deficit assigning to the Rough Below-Falls female × Rough Above-Falls male cross. Smolts leaving fresh water (sampled at two sea-entry traps) were assigned disproportionately to crosses involving one or two Erriff parents. Offspring from pure Burrishoole crosses were more likely to become putative spawners than those from crosses involving one or two Erriff parents, pointing toward possible local adaptation. These results are consistent with heritable variation in migratory tendencies-a key aspect of intraspecific biodiversity that warrants protection-and with previous suggestions that the Burrishoole system may have evolved recently toward reduced anadromy following a novel and catastrophic anthropogenic change.

Keywords: facultative; fitness; microsatellite; migratory; partial migration; smolt; tactic.

FIGURE 1

Map of the Burrishoole and Erriff catchments, western Ireland. The common garden experiment was conducted in the Rough River (highlighted in blue) within the Burrishoole catchment, in a section bounded upstream by the waterfall and downstream by the Rough River Downstream Trap (RRDT). The Rough River connects to Lough Feeagh via the Black River (BR). Trapping facilities at the outlet of Lough Feeagh (MR, Mill Race; SL, Salmon Leap) allowed for monitoring of smolts leaving fresh water. Broodstock for the experiment were obtained from above and below the waterfall in the Rough River (Burrishoole catchment) and from various sites throughout the Erriff catchment. Sea trout smolts and kelts are monitored at the Tawnyard Trap (TT) in the Erriff system, just downstream of Lough Tawnyard.

FIGURE 2

The proportions of Salmo trutta from the various samples assigned to each experimental cross type. EF, electrofishing. RRDT, Rough River Downstream Trap.

FIGURE 3

Histograms showing counts of Salmo trutta per month, broken down by experimental cross, for the various samples. RRDT, Rough River Downstream Trap.

FIGURE 4

Fork length of parr sampled by electrofishing in (a) the Rough River or (b) the Rough River Downstream Trap (RRDT) between 2017 and 2019 as a function of cumulative growing degree days (CGDD). The regression lines and associated 95% confidence intervals are included for visual purposes only, as the underlying analysis was based on log‐transformed fish size. The relationships were approximately linear on the untransformed scale also, however.