Increased numbers of great cormorants (Phalacrocorax carbo) in the Baltic Sea may have local impacts on fisheries and salmonid hatcheries. We studied spatial and temporal variability in cormorant diet, and potential consumption of hatchery salmonids, by analysing knee subcutaneous adipose tissue fatty acids (FA) of specimens (N = 77) collected along Swedish and Finnish coasts in different seasons during 2013–2017. The FA profiles of the subspecies sinensis and carbo were similar, with large individual variation. The proportion of C18 polyunsaturated FAs (PUFAs) was the largest in the north, whereas the proportion of C20–22 monounsaturated FAs (MUFAs) increased towards the south, reflecting diminishing freshwater and increasing marine food web characteristics towards the south. As an exception, the C20–22 MUFA percentage was high in sinensis collected in June 2017 from the northern Baltic Sea. The source of C20–22 MUFAs was probably hatchery salmonids, raised on ocean fish hatchery feed and released 10 days before, near the cormorant capture site. The FA profiles of northern and southern cormorants differed from each other both in early and late summer samples, suggesting spatially different diets. The largest individual variation was found in 22:1n-11, characteristic of ocean zooplanktivorous fish, and likely originating from Atlantic wild or Baltic Sea hatchery-reared fish. This study shows that adipose tissue FA profiles can be used as proxies for seabird diet monitoring and indicators of predation on hatchery-reared fish. Obtaining quantitative estimates on the proportions of dietary fish species requires future feeding experiments, allowing calibration between the FA compositions and diet.

中文翻译：

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脂肪组织脂肪酸表明波罗的海地区大鸬鹚的时空饮食差异

大鸬鹚数量增加（Phalacrocorax carbo) 在波罗的海可能对渔业和鲑鱼孵化场产生局部影响。我们通过分析 2013-2017 年不同季节在瑞典和芬兰海岸采集的样本 (N = 77) 的膝部皮下脂肪组织脂肪酸 (FA)，研究了鸬鹚饮食的空间和时间变异性，以及孵化场鲑鱼的潜在消费量。亚种的FA概况中华和碳水化合物相似，个体差异较大。C18 多不饱和脂肪酸（PUFAs）的比例在北部最大，而 C20-22 单不饱和脂肪酸（MUFAs）的比例向南增加，反映出向南淡水减少和海洋食物网特征增加。作为一个例外，C20-22 MUFA 百分比在中华于 2017 年 6 月从波罗的海北部采集。C20-22 MUFA 的来源可能是孵化场鲑鱼，它们在海洋鱼类孵化场饲料中饲养并在 10 天前在鸬鹚捕获地点附近释放。在夏初和夏末样本中，北部和南部鸬鹚的 FA 剖面彼此不同，表明在空间上存在不同的饮食。在 22:1n-11 中发现了最大的个体变异，这是海洋浮游动物鱼类的特征，可能起源于大西洋野生或波罗的海孵化场饲养的鱼类。这项研究表明，脂肪组织 FA 谱可用作海鸟饮食监测的代理和孵化场饲养鱼类捕食的指标。获得对膳食鱼类比例的定量估计需要未来的喂养实验，