J. Mark Hanson

Evolutionary response to size-selective mortality in an exploited fish population

Many collapsed fish populations have failed to recover after a decade or more with little fishing. This may reflect evolutionary change in response to the highly selective mortality imposed by fisheries. Recent experimental work has demonstrated a rapid genetic change in growth rate in response to size-selective harvesting of laboratory fish populations. Here, we use a 30-year time-series of back-calculated lengths-at-age to test for a genetic response to size-selective mortality in the wild in a heavily exploited population of Atlantic cod (Gadus morhua). Controlling for the effects of density- and temperature-dependent growth, the change in mean length of 4-year-old cod between offspring and their parental cohorts was positively correlated with the estimated selection differential experienced by the parental cohorts between this age and spawning. This result supports the hypothesis that there have been genetic changes in growth in this population in response to size-selective fishing. Such changes may account for the continued small size-at-age in this population despite good conditions for growth and little fishing for over a decade. This study highlights the need for management regimes that take into account the evolutionary consequences of fishing.

Effects of impoundment and water-level fluctuations on macrophyte and macroinvertebrate communities of a dammed tidal river

A freshwater headpond was created in 1968 when a causeway was builtacross a tidal portion of the Petitcodiac River, New Brunswick, Canada. In the32-year history of this tidal barrier, there have been numerous water-leveldrawdowns to manage risk of flooding (relatively minor and of short duration)and to permit passage of anadromous fishes past the dam structure (often ofseveral weeks duration). Following five years of routine management to preventflooding, there were three major water-level drawdowns that fully exposed thelittoral zone to air for varying lengths of time during the spring of 1997 (7days), 1998 (19 days), and 1999 (55 days). The 1998 and 1999 drawdowns resultedin significant decreases in biomass of macrophytes and benthicmacroinvertebrates in the littoral and sublittoral zones, compared to 1997. Thelargest decrease occurred between 1997 and 1998 when populations of mostspecieswere largely eliminated. The remaining species were those resilient toprolongedexposure to the atmosphere. The only macrophyte to increase in biomass over thethree years was Polygonum lapathifolium, which formedimpenetrable mats in 1999. Hexagenia limbata in thesublittoral zone was the only benthic invertebrate species to increase inbiomass. The age distribution of the eastern floater (Pyganodoncataracta) population suggested that a major drawdown accompanied bysalt-water intrusion had eliminated the entire freshwater community in 1988.Thegoals of allowing passage of anadromous fishes past the dam structure and thedevelopment of a stable, artificial, freshwater-lake community appear to beincompatible.

Seasonal use of estuaries by winter flounder in the Southern Gulf of St. Lawrence

Abstract The seasonal distribution of winter flounder Pleuronectes americanus in the southern Gulf of St. Lawrence was examined by analyzing data from research surveys conducted from 1986 to 1994 in the southern Gulf and from beach seining, trawling, and sampling of commercial fisheries in the Miramichi Estuary. During June–August, most winter flounder were found in coastal waters (<40 m deep) and within a temperature range of 3–16°C. Abundance in coastal waters declined sharply between September and late November–early December. Winter flounder were not captured in deep offshore waters during winter; rather, large numbers moved into the Miramichi Estuary during October and November and remained until spring. The brackish water under the ice of the Miramichi Estuary was warmer (–1 to 0°C) than the full salt water under the ice of the southern Gulf (below –1.5°C) and could provide a thermal refuge for winter flounder as well as protection from ice contact during winter storms. Adult winter flounder left th...

THE DAMMING OF THE PETITCODIAC RIVER: SPECIES, POPULATIONS, AND HABITATS LOST

Abstract Construction of the Petitcodiac River Causeway in 1968 converted 21 km of macrotidal (tidal amplitude > 8 m) brackish-water estuary into a shallow impoundment. The ecological value of the freshwater impoundment was low, when judged by criteria of temporal stability, uniqueness, conservation of native species, and level of disturbance. By interfering with migrations, the causeway dam eliminated or greatly reduced the populations of most diadromous fishes, including a large population of the now-endangered Inner Bay of Fundy stock of Atlantic salmon (Salmo salar). Loss of the host fish for the parasitic glochidial stage resulted in extirpation of the dwarf wedgemussel (Alasmidonta heterodon) from Canada. Benthic and planktonic communities of the impoundment were depauperate, and typical of a disturbed ecosystem. Most species were of marine/estuarine origin, or belong to freshwater groups with rapid dispersal. Illegally introduced warm-water fishes flourished in the impoundment, presumably replacing the native species.

Evolution of growth in Gulf of St Lawrence cod: reply to Heino et al.

In a recent paper ([Swain et al . 2007][1]), we examined the evidence for a genetic response to size-selective mortality in southern Gulf of St Lawrence cod, using data on back-calculated length at age and a quantitative genetics model. We used the model![Formula][2] (1.1)where Δ L 4 and Δ E