1. Determining the comparative impacts of increased intra‐ versus inter‐specific competition is important in freshwater ecosystems for understanding the ecological changes that can result from activities such as fish stocking events (using alien and/or native fish species), as well as from natural processes that elevate population abundances (e.g. increased annual recruitment success). While increased inter‐specific competition can result in slower growth rates and/or reduced population density in the weaker or less abundant competitor, it is important that this is assessed in relation to the impacts of increased intra‐specific competition.
2. We tested how the strength of inter‐specific competition from a co‐existing species varies with abundance, and how this compares with increased intra‐specific competition. Fish were the model taxa, as their growth rates strongly correlate with competitive success. Replicated pond mesocosms (150 days) used chub Squalius cephalus in an allopatric control (n = 5; C5) and allopatric treatment (n = 10; C10), and in sympatric treatments (n = 5) with European barbel Barbus barbus (n = 5 (T1), 10 (T2) and 15 (T3)). Treatment effects were tested on fish‐specific growth rates (SGRs), and the size and position of the trophic and isotopic niche (stomach contents and stable isotope [SI] analyses respectively).
3. Chub SGRs were significantly higher in C5 versus all other treatments, but did not differ among the other allopatric and sympatric treatments. Chub trophic niche sizes in T1 to T3 were significantly smaller than C5, indicating more specialised diets in the presence of barbel. Chub trophic niche size in C10 was, however, larger than C5 and T1, indicating a shift to a more generalised diet as intra‐specific competition increased.
4. As SGRs reduced in treatments, so did the predicted extent of fish SI turnover, with SI data in T1 to T3 not at isotopic equilibrium with their diet in the mesocosms at the experiment's end. Following conversion of fish SI data to represent values at 95% isotopic turnover, chub isotopic niches also revealed shifts to a more general diet as intra‐specific competition increased, but to more specialised diets as inter‐specific competition increased.
5. Increased intra‐ and inter‐specific competition impacts on the trophic and isotopic niches were contrasting; both metrics indicated niche constrictions in sympatry but niche expansions in allopatry. Impacts on fish growth were evident from both. These results emphasise that the trophic consequences of competition in freshwater fish can differ between stocking events involving the release of conspecifics or other species, with this having important considerations for how freshwater fish communities are managed for angling exploitation.

中文翻译：

---

淡水鱼竞争性相互作用的营养后果：密度依赖性效应和种间竞争与种内竞争的影响

1. 确定种间竞争与种间竞争之间的比较影响在淡水生态系统中很重要，因为这有助于了解可能因鱼类种群活动（使用外来和/或本地鱼类）和自然过程等活动而引起的生态变化从而提高了人口数量（例如，增加了年度招聘成功率）。尽管种间竞争加剧会导致较弱或较不富裕竞争者的增长速度降低和/或种群密度降低，但必须根据种内竞争加剧的影响进行评估。
2. 我们测试了共存物种间种间竞争的强度如何随丰度变化，以及与种内竞争增加的比较。鱼是典型的分类单元，因为它们的生长速度与竞争成功密切相关。复制池塘生物群落（150天）用鲐Squalius法罗斯在异域控制（Ñ  = 5; C5）和异域处理（Ñ  = 10; C10），以及在同域处理（Ñ  = 5）欧洲触须鲃（Ñ = 5（T1），10（T2）和15（T3））。测试了对鱼类特定生长率（SGR）以及营养和同位素生态位的大小和位置（分别为胃内容物和稳定同位素[SI]分析）的治疗效果。
3. 与其他所有治疗相比，C5中的Chub SGR显着更高，但在其他同种异体治疗和同伴治疗之间没有差异。T1至T3的丘比营养位显着小于C5，表明在存在通天粉的情况下，饮食更加专门化。但是，C10中的丘比营养位的大小大于C5和T1，表明随着种内竞争的加剧，人们转向了更普遍的饮食。
4. 随着处理过程中SGR的降低，鱼类SI转换的预测程度也随之降低，T1到T3中的SI数据在实验结束时不在中膜的饮食中达到同位素平衡。将鱼类的SI数据转换为代表95％同位素转换率的值后，种同位素生态位也显示出随着种内竞争增加而转向更普通的饮食，但随着种间竞争增加向更特殊的饮食转变。
5. 种内和种间竞争对营养位和同位素位的影响增加形成对比。两种指标均显示了共生中的利基收缩，而异位性中的利基扩张。两种对鱼类生长的影响都是显而易见的。这些结果强调，在涉及放养物种或其他物种的放养事件之间，淡水鱼竞争的营养后果可能会有所不同，这对于如何管理淡水鱼群落进行垂钓有重要考虑。