Habitat complexity influences the growth rate of juvenile gadiformes (gadus morhua, gadus ogac, urophysis tenuis) in newman sound, Newfoundland

Renkawitz, Mark D. (2008) Habitat complexity influences the growth rate of juvenile gadiformes (gadus morhua, gadus ogac, urophysis tenuis) in newman sound, Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

In Newman Sound, Newfoundland, juvenile fish settle in shallow near-shore waters and are often associated with eelgrass beds of intermediate structural complexity. Although it is well established that structurally complex habitats such as eelgrass provide a refuge for juvenile fish from larger predatory fish, little is known about the potential energetic reward associated with use of these complex habitats. The settlement and close association of age 0 and 1 juvenile fish (approximately 60-100 mm SL) with eelgrass habitat may be the result of an active compromise in which optimal foraging habitat is sacrificed for habitats with increased shelter from predators. In this study, I quantified the relative growth rates of fishes associated with three adjacent habitats of differing structural complexity (barren seafloor, eelgrass, and water column) at five sites in Newman Sound, Newfoundland. Juvenile Greenland cod (Gadus ogac), Atlantic cod (G. morhua), and white hake (Urophysis tenuis) were placed in 1 m³ enclosures positioned over eelgrass, barren seafloor, and water column habitats in 2002 and 2003. Changes in standard fish length (mm SL) and volume (ml) were measured, and specific daily growth rates were determined and compared. Stomach contents were examined for habitat-related differences in the type and quantity of items consumed by enclosed fish at the termination of each experiment. Zooplankton samples were also collected biweekly during summer and fall in 2003 to determine if differences in prey concentration differed among the habitats. -- Annual and seasonal variations in growth rates were documented among habitats and between the species. In fall 2002, there was no statistical difference in specific growth rates (SGR) of Greenland cod among the habitats (barren = 0.068 % SL·day⁻¹, eelgrass = 0.074 % SL·day⁻¹, water column = 0.064 % SL·day⁻¹). SGR of Atlantic cod during winter from 2002 to 2003 did not differ significantly either (barren = 0.129 % SL·day⁻¹, eelgrass = 0.151 % SL·day⁻¹, deep water = 0.116 % SL·day⁻¹), but survival was significantly greater in deeper habitats (55 %) than in shallower habitats (20 %). In spring 2003, mean SGR (± SE) of Atlantic cod was significantly greater in eelgrass than barren seafloor or water column habitats (0.366 ± 0.026 % SL·day⁻¹, 0.327 ± 0.035 % SL·day⁻¹, and 0.065 ± 0.013 % SL·day⁻¹ respectively). In summer 2003 juvenile white hake grew more rapidly in eelgrass (0.713 ± 0.062 % SL·day⁻¹) than in barren or water column habitats (0.483 ± 0.055 % SL·day⁻¹ and 0.271 ± 0.040 % SL·day⁻¹ respectively). In fall 2003, juvenile Greenland cod grew more rapidly in water column (0.449 ± 0.055 % SL·day⁻¹) habitats than in either barren (0.372 ± 0.028 % SL·day⁻¹) or eelgrass habitats (0.254 ± 0.013 % SL·day⁻¹). Diets were similar among habitats within experiments, but differed over time. Fish in eelgrass tended to have greater amount of food by weight in their stomachs at the time of sampling than fish in either barren or water column habitats (1.03 %, 0.88 % and 0.69 % respectively). Gadiformes in each habitat appeared to select for benthic or epibenthic prey. The concentration of available prey differed among the habitats. Eelgrass samples had the highest concentration of zooplankton (1.93 individuals·Liter⁻¹), followed by barren (1.33 individuals·Liter⁻¹) and water column samples (0.99 individuals·Liter⁻¹). -- These data suggest that at certain times of the year, juvenile fish settle and occupy structurally complex habitats for energetic reward as measured by growth. At the scale of these experiments, there were differences in the growth rates, food availability and zooplankton concentration between three habitats at my sites. Enclosure methodology can be a useful tool in determining relative differences between specific juvenile fish foraging habitats provided that confounding variables and artifacts of the experimental method are rigorously accounted for. Understanding the relationships between specific habitat components (e.g., vegetation) and fish growth aids our understanding of juvenile fish ecology, and may ultimately help restore depleted fish populations in the Northwest Atlantic through habitat conservation and protection.

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