We examined shelter occupancy and behavioural interactions in non-reproductive male round gobies, Neogobius melanostomus, an invasive fish, to determine if gobies can assess one another's prowess effectively. Results of laboratory experiments revealed a significant, positive relationship between body size (total mass) and time for individual fish to occupy shelters. Shelter size selection did not vary with body size, but males that abandoned shelters were smaller than those that remained. Overall, the number of interactions between residents and intruders were low with most interactions occurring between large residents and large intruders. A size differential between residents and intruders of 3% was sufficient to predict the proportion of victories in conflicts between round gobies. This ability of the round goby to perceive size differences between themselves and potential opponents reduces the number of interactions and may account for the occurrence of high densities of round gobies observed in the field.

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Open agricultural drains (ditches) provide necessary drainage for cropland and may also provide habitat supporting native fish assemblages. We tested whether warm-water drains in southwestern Ontario provide fish habitat similar to that of reference watercourses not subjected to drain maintenance. Features of fish assemblages and habitats were characterized using standardized protocols in 24 pairs of drains and reference watercourses. Drains and reference watercourses did not differ significantly in fish species richness, evenness, abundance, biomass, species composition, life stages, or nine of 13 physical habitat attributes expected to be altered by drain maintenance. These findings do not support concerns that drain maintenance has strong, lasting effects on fish assemblages and physical habitat features. Other landscape-level agricultural impacts may affect both drains and reference watercourses but were not examined in this study. We recommend that drains continue to be recognized as fish habitat under the Canadian Fisheries Act and that drain and fish habitat managers strive for logical, scientifically defensible drain maintenance practices that preserve fish biodiversity and habitat, while considering the needs of agriculture.

Greenhouse production of vegetables is a growing global trade. While greenhouses are typically captured under regulations aimed at farmland, they may also function as a point source of effluent. In this study, the cumulative impacts greenhouse effluents have on riverine macronutrient and trace metal concentrations were examined. Water samples were collected Bi-weekly for five years from 14 rivers in agriculturally dominated watersheds in southwestern Ontario. Nine of the watersheds contained greenhouses with their boundaries. Greenhouse influenced rivers had significantly higher concentrations of macronutrients (nitrogen, phosphorus, and potassium) and trace metals (copper, molybdenum, and zinc). Concentrations within greenhouse influenced rivers appeared to decrease over the 5-year study while concentrations within non-greenhouse influenced river remained constant. The different temporal pattern between river types was attributed to increased precipitation during the study period. Increases in precipitation diluted concentrations in greenhouse influenced rivers; however, non-influenced river runoff proportionally increased nutrient mobility and flow, stabilizing the observed concentrations of non-point sources. Understanding the dynamic nature of environmental releases of point and non-point sources of nutrients and trace metals in mixed agricultural systems using riverine water chemistry is complicated by changes in climatic conditions, highlighting the need for long-term monitoring of nutrients, river flows and weather data in assessing these agricultural sectors.