Evidence for density-dependent mortality in recruitment of a temperate reef fish, cunner Tautogolabrus adspersus, among similar reefs in the vicinity of an anthropogenic disturbance

Abstract

Determining the factors that regulate recruitment of cunner Tautogolabrus adspersus, a temperate reef fish, can help assess the effect of an anthropogenic disturbance on population abundance. The relative abundance of cunner recruits was tracked over 1 reproductive season via visual counts on line transects across similar habitats at 4 sites across 3 boulder reefs in the vicinity of the Pilgrim Nuclear Power Station (PNPS) in Plymouth, Massachusetts, USA. Habitat characteristics including macroalgae structure, canopy height, rugosity (a measure of structure complexity), temperature, and visibility were measured at each site to assess the influence of these factors on recruit abundance. We assumed that immigration and emigration were not responsible for recruit abundance patterns through time due to the sedentary life history of cunner recruits. We observed differences in recruit abundance over the settlement period among reefs which were likely the result of differences in settlement. However recruitment at the end of the post-settlement period was similar across reefs. Recruit mortality rates among reefs differed in the post-settlement period, with the reef that had the highest density also having the highest mortality rate. Post-settlement mortality rates provided evidence for density-dependent mortality, since habitat and abiotic factors failed to explain the difference in mortality among reefs. Consequently, because of the existence of density-dependent processes in the post-settlement stages, entrainment mortality of the pre-settlement egg and larval stages by the PNPS will have a diminished effect on the local cunner population. An additional assessment of within-reef variability in recruitment also suggests recruitment success is sensitive to small changes in habitat structure which can occur over the recruitment season.

Interactions between climate and population density in the episodic recruitment of bocaccio, Sebastes paucispinis, a Pacific rockfish

“…Thus, periods of above‐average growth may allow females to invest more energy in reproduction (Harvey, 2005), either producing more or larger eggs, perhaps resulting in the ‘carry over’ effect we detected. Our observation that density‐dependent mortality appeared to occur during the settlement period was consistent with experimental research on population dynamics in demersal fishes on temperate and coral reefs showing that, at least locally, density‐dependent mortality occurs in the period shortly after settlement (Hixon and Carr, 1997, Anderson, 2001; Nitschke et al. , 2002; Hixon and Jones, 2005, Johnson, 2006).…”

Role of habitat in mediating mortality during the post‐settlement transition phase of temperate marine fishes

“…This result led them to conclude that habitat‐mediated post‐settlement processes are critical to recruitment and population dynamics of cunner. In a similar study in coastal Massachusetts, Nitschke et al (2002) found strong evidence for density‐dependent mortality in the post‐settlement phase and that small changes in habitat structure across reefs over the recruitment season can lead to variability in recruitment. Thus, reefs that decreased in complexity experienced higher cunner mortality.…”

Foraging arena theory

“…In such cases, there can be strong density‐dependent increase in agonistic activity with increasing density of conspecifics, leading to increased predation risk and density‐dependent mortality at high densities. Indeed, one of the first attempts to test foraging arena predictions involved a small benthic fish (cunner, Tautogolabrus adspersus ), which has exhibited exactly such increase in activity and mortality when its densities were experimentally manipulated or varied among reef sites (Nitschke et al. 2002; Juanes 2005).…”