Summary
The energetic and physiological status of parental smallmouth bass Micropterus dolomieu was investigated across the majority of their latitudinal range at the onset and near the end of care. Variables such as tissue lipid stores, plasma indicators of nutritional status and chronic stress and white muscle were used to define energetic and physiological status. Results showed that northern males (48° N) were larger and heavier than mid‐northern (44° N) and southern (36° N) latitude males. For a given body size, northern males had greater whole‐body lipid across the parental care period and tended to feed more (based on gut contents) than mid‐northern and southern latitude conspecifics. Indicators of nutritional status were also highest in northern males. Conversely, the southern males exhibited the greatest capacity for biosynthesis across the entire parental care period as indicated by the highest level of nucleoside diphosphate kinase activities. Collectively, these finding suggest that the energetic costs and physiological consequences of care vary across latitudes, providing some of the first mechanistic evidence of how environmental conditions can influence both the ecological and physiological costs of reproduction for wild animals during parental care. The data also suggest that lake‐specific processes that can vary independently of latitude may be important, necessitating additional research on fish reproductive physiology across landscapes.
Methodology
Study sites and subjects
Male M. dolomieu were collected from five lakes across North America [Bull Shoals Lake, MO–AR, U.S.A. (36° 25′ N; 92° 45′ W) and Table Rock Lake, MO, U.S.A. (36° 35′ N; 93° 50′ W) 20 April to14 May 2007, Charleston Lake (44° 32′ N; 76° 00′ W) and Big Rideau Lake, Ontario, Canada (44° 46′ N; 76° 13′ W) 25 May to 9 June 2007, and Rainy Lake, Ontario, Canada (48° 38′ N; 93° 15′ W), 8 to 21 June 2007] (Fig. 1). These locations were selected in order to represent the extremes of the latitudinal range of M. dolomieu (Scott & Crossman, 1973; Mandrak & Crossman, 1992). Average winter temperature (from December to March) at 36° 21′ N is 4·1° C , −6·1° C at 44° 44′ N and −10·8° C at 48° 43′ N [National Oceanic and Atmospheric Administration (NOAA); http://www.ncdc.noaa.gov/normals.html]. Lakes at the most southern latitude do not experience ice‐cover and these areas receive only trace amounts of snow (NOAA). Associated with latitude are many other factors that could influence the ecology and chemistry of the different lakes including geology, productivity, fish community and food web structure. It is not possible to address all of these factors in this study and they are simply considered to be part of the suite of changes associated with latitude. Experimental mesocosm studies would be needed to tease out the relative influence of these and other factors.
Sampling occurred within a single spawning season (by progressively moving northward) to eliminate inter‐year variability. Fish were sampled at two lakes (with the exception of the northern site) in order to provide some replication at a given latitude. For the purpose of analyses these lakes were considered separately as there were insufficient lake replicates at each latitude to include latitude as a main effect in the statistical models. Scientific collection permits were obtained from the relevant natural resource agency (Missouri Department of Conservation and Ontario Ministry of Natural Resources) and animal care protocols were approved by the Canadian Council on Animal Care. Upon arrival at each study site, snorkelling surveys were completed in order to identify M. dolomieu nests (n = >30) with fresh eggs. Water temperatures were between 14·8 and 15·5° C at all locations when nests were identified and when fish were sampled on fresh eggs. Nests were marked with labelled polyvinyl chloride (PVC) tiles that were removed at the end of parental care. Among these nests, 10 were randomly selected and the males were angled using heavy fishing equipment and landed in <30 s. Previous studies have revealed that if M. dolomieu are landed and blood sampled in c. <2 min, the physiological changes in the fish associated with exercise and handling are not yet reflected in the blood and reflect the condition of the fish prior to their capture (e.g. cortisol levels are typically barely detectable and at lower levels than can be obtained by holding fish in sensory deprivation chambers; Hanson et al. , 2009). Fish were immediately transferred to a foam‐lined trough filled with fresh lake water for hook removal and blood sampling. Fish were placed in a supine position for blood sampling via caudal venipuncture using a vacutainer (10 ml, lithium–heparin anticoagulant, Becton‐Dickson; 21 G, 1 1/2 syringe; www.bd.com). The filled vacutainer was immediately placed in a water–ice slurry and stored for up to 2 h. Blood was later centrifuged at 10 000 g for 7 min (Clay Adams Compact II Centrifuge, Corning; www.analyticalsci.com). Plasma vials were then placed in liquid nitrogen until transferred to a −80° C freezer until analysis. Fish were euthanized by cerebral percussion immediately after blood sampling. Axial and pectoral muscle samples were taken using a disposable sterile scalpel, wrapped in foil and placed immediately in liquid nitrogen until they could be transferred to a −80° C freezer. Sampling time (from hook removal to muscle being placed in liquid nitrogen) took between 3 and 8 min. Fish were then measured for total length (L T, mm) and the carcass was sealed in a plastic bag (to prevent dehydration) on ice for up to 8 h. They were then transferred to a chest freezer until dissection (up to 4 days).