The effects of increasing habitat complexity with artificial reefs on demersal fish density in coastal Newfoundland Waters

Sargent, Philip S. (2002) The effects of increasing habitat complexity with artificial reefs on demersal fish density in coastal Newfoundland Waters. Masters thesis, Memorial University of Newfoundland.

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Abstract

Abstract Habitat complexity and predator avoidance are important factors influencing the distribution of organisms. Structurally complex habitats offer refuge from predators and potential foraging areas. Artificial reefs increase habitat complexity in the aquatic environment. In this study, artificial reefs were used to test the hypothesis that increased habitat complexity would increase density of demersal fishes, such as cunners (Tautogolabrus adspersus) juvenile Atlantic cod (Gadus morhua) and juvenile rock cod (G. ogac) in the coastal subarctic waters of Newfoundland. In 1999 and 2000, five paired artificial reef and control transects and three additional control transects (each 80 m long) were deployed in Newman Sound, Newfoundland, Canada along the 15 m depth contour. Habitat complexity of substrate along the transects was expressed as fractal dimensions (D) measured at five resolutions (0.035 - 3.5 m). Densities of cunners and juvenile cod were measured on each transect during three autumn surveys in 1999 and four summer surveys in 2000 using scuba. Fractal dimensions were between D = 1.00 and 1.01 along unmanipulated control transects and D = 1.16 along reef transects, indicating a significant increase in habitat complexity due to the artificial reefs. During the day, most cunners (159 of 242) and juvenile cod (25 of 29) were observed on reef transects in close association with artificial reefs. When observed on control transects, cunners associated with unique features of increased complexity. Juvenile cod observed on control transects showed no habitat associations. Densities of cunners, juvenile Atlantic cod, and rock cod attenuated at rates of-1.08, -0.22, and -0.17 %/m respectively with distance from reefs to a "baseline" level at 15-20 m. In summer 2000, higher cunner densities were associated with increased habitat complexity during the day and few cunners (2) were observed at night. In contrast, no juvenile Atlantic cod were observed over any transects during the day, but at night, density increased, though no preference for complexity was observed. Juvenile rock cod were observed in similar densities during day and night, but individuals were aggregated near the reefs during the day and were dispersed at night. Cunners were associated with artificial reefs in both summer and autumn. Cunners were observed less frequently at low temperatures. Juvenile Atlantic cod associated with artificial reefs in autumn but not in summer. In contrast, juvenile rock cod associated with reefs in summer but not in autumn. Seasonal differences in habitat use by juvenile cod appeared to be linked to the breakdown in the thermocline in early autumn. Artificial reefs increased habitat complexity and in turn the diel and seasonal distribution and density of demersal fish species.

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