Indirect Contramensalism Through Trophic Cascades Between Two Omnivorous Fishes

Abstract

The impact of trophic cascades on the interspecific relationship between two omnivorous stream fish, ayu (Plecoglossus altivelis altivelis) and Japanese dace (Tribolodon hakonensis), were analyzed in experimental pools. Benthic algae grew on the pool beds, and macroinvertebrates colonized the pools. Ayu predominantly grazed the algae, occasionally forming territories. Japanese dace fed on both benthic algae and invertebrates when ayu were absent, and predominantly on invertebrates when ayu were present. The presence of Japanese dace reduced the number of invertebrates and increased algal biomass (chlorophyll a) through trophic cascades, but the effects of respiration and excretion of dace did not notably increase algal biomass probably because the water continuously pumped into the pools was already nutrient rich. The growth rate of ayu was enhanced in the presence of Japanese dace due to increased algal biomass. In contrast, as the number of ayu increased, the growth rate of Japanese dace was reduced because of the exploitation of benthic algae and occasional attacks by ayu on dace. The relationship between ayu and Japanese dace is not merely competitive, but partly indirect contramensalism through trophic cascades. The occasional attacks by ayu on Japanese dace were mainly directed at individuals moving near the pool beds and therefore might function to increase benthic algal resources by limiting grazing by dace.

Effects of experimental duration and density of Japanese dace Tribolodon hakonensis on the strength of trophic cascades on benthic algae

“…Therefore, ayu might restrict dace movement and grazing behavior to some extent to defend algal resources against dace. Occasional attacks by ayu on other Wsh, such as Japanese dace and pale chub Zacco platypus, have also been reported in previous studies (Katano and Aonuma 2001;Katano et al 2003). There was interactive and exploitative competition between dace and ayu, although ayu were dominant over dace in the aggressive encounters and in the outcome of indirect eVects.…”

“…3 Abundance of chlorophyll a, number of benthic macroinvertebrates per 25 £ 25-cm plate, and number of gastropods and ephemeropteran nymphs per plate in the dace density-algal biomass experiment conducted over 8 days. Vertical bars represent standard error (SE) and n = 4 for each treatment and subsequently ayu growth through trophic cascades in the 20-day experiment, as reported in the previous studies (Katano et al 2003(Katano et al , 2006. Twenty dace, which were fed ad libitum, in a circular stainless cage did not have a facilitative eVect on algae in other parts of the pools.…”

“…I showed in previous studies that the presence of omnivorous Japanese dace Tribolodon hakonensis increases the algal biomass through trophic cascades by preying on algivorous invertebrates, which then enhances the growth rate of ayu Plecoglossus altivelis altivelis, which mainly grazes benthic algae (Katano et al 2003). In this study, I controlled the density of dace to examine the changes in the intensity of trophic cascades on algal biomass, ayu growth, and dace foraging tactics.…”

“…To test the eVects of respiration and excretion of dace on algal biomass, we set a circular stainless steel cage (30-cm diameter and 56-cm high) with 20 dace and a mesh size of 1 mm near (<20 cm) the center of each of four randomly selected pools. Dace in cages were fed ad libitum aquatic insects (>1 mm) and food pellets, as detailed in Katano et al (2003). The experiment was conducted over 20 days from 14 June to 4 July 2005 (20-day experiment).…”

“…Therefore, the presence or absence of ayu may lead to diVerent stable community structures and environmental conditions for trophic cascades, although trophic cascades were detected in both communities with and without ayu in the present study. The presence of ayu reduces the growth rate of other Wsh, such as dace (Katano et al 2003) and pale chub (Katano and Aonuma 2001). Trophic cascades triggered by dace and other omnivorous Wsh aVect algal biomass and ayu growth, and ayu have great eVects on various components in a community.…”