The fish community of Hamilton Harbour, Lake Ontario: Status, stressors, and remediation over 25 years

Hamilton Harbour is a large (21 km2) protected harbour located at the western end of Lake Ontario that was designated as an Area of Concern in 1985 by the International Joint Commission. As part of the designation, the fish community was deemed “impaired.” One of the long-term goals of the Hamilton Harbour Remedial Action Plan is to rehabilitate the fishery to a state, at least in part, to what it was prior to the degrading effects of industrial and municipal pollution, habitat loss, and invasive species. Since the Area of Concern designation, the nearshore fish community has been monitored regularly via a federal boat electrofishing program (1988–2013) and more recently, by a trap net program (2006–2012). During the study period, the harbour underwent significant physical and biological change which was related to both lake-wide (e.g. Dreissenids) and localized harbour events (e.g. habitat restoration). The fish community was assessed temporally using two indices of ecosystem health, the Great Lakes Index of Biotic Integrity and Proportion of Piscivore Biomass, and species-specific catch trends at unaltered sites or sites modified by habitat restoration. Early on, the fish community was dominated by tolerant species characteristic of degraded, eutrophic environments and piscivore abundance was low. The fish community responded positively to ecosystem actions during the first decade that improved water quality, increased physical habitat and reduced invasive species, but further progress was confounded by the interactions with other factors in the last decade that included new invasive species and a decline in water quality. Fish community assessments in the last decade, using both the indices found that the fish community was still impaired and dominated by non-native and pollution tolerant species. The Index of Biotic Integrity was a suitable indicator of ecosystem health strengthened by a consistent assessment with the Proportion of Piscivore Biomass index. A science based refinement of the existing Index in the future would benefit the ability to assess nearshore fish assemblages in the context of conservation goals.汉密尔顿港湾是一处面积达21平方千米的受保护港湾，坐落于安大略湖西端，1985年被国际联合委员会（International Joint Commission）列为关注区域（Area of Concern）。该区域被列入名录时，其鱼类群落被认定为“受损”。《汉密尔顿港湾补救行动计划》（Hamilton Harbour Remedial Action Plan）的长期目标之一，是将渔业恢复至工业与城市污染、栖息地丧失以及外来入侵物种（invasive species）造成退化之前的状态，至少部分恢复至彼时水平。 自该区域被列为关注区域以来，研究人员通过联邦船只电捕鱼项目（boat electrofishing program，1988—2013年）以及近期的陷阱网项目（trap net program，2006—2012年）对近岸鱼类群落（nearshore fish community）开展了常态化监测。本研究周期内，港湾经历了显著的物理与生物变化，此类变化既与全湖尺度的生态事件（如斑贝类（Dreissenids）入侵）相关，也与港湾本地的生态修复事件（如栖息地修复（habitat restoration））有关。 研究团队通过两项生态系统健康指数——五大湖生物完整性指数（Great Lakes Index of Biotic Integrity）与食鱼鱼类生物量占比（Proportion of Piscivore Biomass）——以及未受干扰位点或经栖息地修复位点的物种特异性捕捞趋势（species-specific catch trends），对鱼类群落开展了时间序列评估。研究初期，鱼类群落以适应退化富营养化环境（eutrophic environments）的耐污物种为优势类群，食鱼鱼类（piscivore）的种群丰度较低。 在修复行动实施的首个十年间，随着水质改善、物理栖息地扩增以及外来入侵物种防控取得成效，鱼类群落呈现出积极的恢复态势；但在研究周期的最后十年，新增外来入侵物种与水质下滑等多重因素的交互作用，使得修复进程的进一步推进受到了制约。基于上述两项指数开展的近十年鱼类群落评估结果显示，该港湾的鱼类群落仍处于受损状态，仍以外来物种与耐污物种为优势类群。 生物完整性指数可作为生态系统健康的有效指示指标，结合食鱼鱼类生物量占比指数开展的一致性评估进一步验证了该结论的可靠性。未来对现有指数开展基于科学依据的优化完善，将有助于在生态保护目标框架下更精准地评估近岸鱼类群落结构（fish assemblages）。