Summary1. Diel vertical migration (DVM) is a widespread phenomenon among marine and freshwater organisms and many studies with various taxa have sought to understand its adaptive significance. Among crustacean zooplankton and juveniles of some fish species DVM is accepted widely as an antipredator behaviour, but little is known about its adaptive value for relatively large-bodied, adult predatory fish such as sharks. Moreover, the majority of studies have focused on pelagic forms, which raises the question of whether DVM occurs in bottom-living predators. 2. To investigate DVM in benthic predatory fish in the marine environment and to determine why it might occur we tracked movements of adult male dogfish ( Scyliorhinus canicula ) by short-and long-term acoustic and archival telemetry. Movement studies were complemented with measurements of prey abundance and availability and thermal habitat within home ranges. A thermal choice experiment and energy budget modelling was used to investigate trade-offs between foraging and thermal habitat selection. 3. Male dogfish undertook normal DVM (nocturnal ascent) within relatively small home ranges ( ∼ 100 × 100 m) comprising along-bottom movements up submarine slopes from deeper, colder waters occupied during the day into warmer, shallow prey-rich areas above the thermocline at night. Few daytime vertical movements occurred. Levels of activity were higher during the night above the thermocline compared to below it during the day indicating they foraged in warm water and rested in colder depths. 4. A thermal choice experiment using environmentally realistic temperatures supported the field observation that dogfish positively avoided warmer water even when it was associated with greater food availability. Males in laboratory aquaria moved into warm water from a cooler refuge only to obtain food, and after food consumption they preferred to rest and digest in cooler water. 5. Modelling of energy budgets under different realistic thermal-choice scenarios indicated dogfish adopting a 'hunt warm − rest cool' strategy could lower daily energy costs by just over 4%. Our results provide the first clear evidence that are consistent with the hypothesis that a benthic marine-fish predator utilizes DVM as an energy conservation strategy that increases bioenergetic efficiency.

No abstract

The impacts of pH and dissolved organic carbon (DOC) on the acute toxicity of Cu to larval fathead minnow (Pimephales promelas) were determined using natural soft water from two Precambrian Shield lakes in south-central Ontario. By artificially manipulating the pH and DOC levels of the water, we demonstrated that both acidification and the removal of DOC increased the toxicity of Cu. The 96-h Cu LC50s were determined over a pH range from 5.4 to 7.3 and a DOC concentration range from 0.2 to 16 mg∙L−1. The LC50s ranged from a low of 2 μg∙L−1 (pH 5.6, DOC 0.2 mg∙L−1) to a high of 182 μg∙L−1 (pH 6.9, DOC 15.6 mg∙L−1). A multiple regression model (log1096-h Cu LC50 = −0.308 + 0.192 pH + 0.136 (pH∙log10DOC)) was used to describe the relationship between Cu toxicity, pH, and DOC. The model was significant (p < 0.00001) and explained 93% of the variability in the toxicity data. These results suggest that current water quality objectives for Cu, and possibly for other metals, may not be sufficiently protective of aquatic life in soft, moderately acidic water containing low levels of DOC.

The effect of substrate surface roughness on small-scale patchiness and the ability of molluscan grazers to feed on intertidal biofilms was examined in a factorial experiment. Granite slabs were treated to create 4 different levels of surface roughness, and biofilm and macroalgae were allowed to recruit. Biofilm cover varied greatly with slab roughness, and was spatially patchy at a scale of millimetres. Diatoms dominated the biofilm, but were less abundant on surfaces with the smallest pits. Cover of diatoms and cyanobacteria was affected by surface roughness, with increased abundance around surface features. Different species of grazer varied in their success at removing certain diatoms and cyanobacteria from slabs of varying roughness, due to either the morphology of the different food types or grazer radula structure. Cover of macroalgal species on the slabs of different roughness also varied, and one species, Hypnea sp., did not recruit on smooth slabs. Rock roughness, therefore, affects both the biofilm and algal species that recruit and their abundance. Grazers were able to remove algae from slabs of all roughness with no apparent species-specific differences in their ability. However, grazer species appear to be more or less efficient at feeding according to the level of roughness, and this combination of variation in rock roughness and grazer efficiency may explain the observed small-scale patchiness on rocky shores in Hong Kong.

Laboratory exposures over 1.3 generations showed that trace metal mixtures played a dominant role in the reproductive failure of American flagfish (Jordanella floridae) in soft (6.0 mg∙L−1 as CaCO3 total hardness), acidified (pH 5.8) water. This finding may also apply for native fish species inhabiting culturally acidified waters. Reproductive failure was complete when pH 5.8 water contained a mixture of Al, Mn, Fe, Ni, Zn, Cu, and Pb at 57% of the concentrations found in acidified waters (or 0.57 ALC). Fry died within 6 d of exposure, adults transferred into the treatment ceased spawning, and hatching of transferred eggs was reduced. The onset of steady spawning activity was delayed by addition of smaller amounts (0.09–0.27 ALC) of metals, but reductions in size of juvenile fish did not persist to maturity. At pH 5.8 with no metals, no effects on any stage of the life cycle were observed. Subsequent testing of fry showed that lethality of a mixture of Al, Zn, and Cu was equivalent to that of all seven metals. The threshold acute LC50 for flagfish fry at pH 5.8 occurred with the simultaneous presence of Al, Zn, and Cu at 29, 5, and 2.3 μg∙L−1, respectively, or with 95 μg Al∙L−1 alone. Future research on trace metal stress in culturally acidified waters should consider Zn and Cu, in addition to Al.