Japanese dace (
Pseudaspius hakonensis
) foraging and predator avoidance behaviors were studied in dace schools with and without vicinity predators in Lake Nojiri, Nagano Japan. Dace and invasive ...smallmouth bass (
Micropterus dolomieu
), which were the only predator observed, were found interacting in shallow habitat along the shoreline. Dace exhibited remarkably different total foraging, foraging modes, and predator avoidance behavior when smallmouth bass were in vicinity. Total foraging was significantly reduced, foraging mode shifted from predominantly benthic foraging to water column foraging, and dace schooled tighter indicated by a significant decrease in nearest neighbor distance and increase in neighbor alignment. These findings show that in Lake Nojiri, dace recognize smallmouth bass as a predator and utilize predator avoidance behaviors. However, the severely decreased total dace foraging when smallmouth bass are in vicinity indicates that even when direct predation is avoided, the indirect effects of reduced foraging and heightened vigilance likely negatively impact dace growth and survival.
Habitat use and diet composition of invasive smallmouth bass,
Micropterus dolomieu
, Japanese dace,
Pseudaspius hakonensis
, and pale chub,
Opsariichthys platypus
, were studied for two years in the ...Middle Chikuma River, Nagano, Japan. Habitat use of dace and chub overlapped both years but showed differences from smallmouth bass. Additionally, smallmouth bass habitat use differed greatly between years, as young smallmouth bass were found in uncharacteristically fast flowing habitat in 2018 and predominantly in more typical slow flowing habitat in 2019. This use of fast flowing habitat in 2018 appears to be influenced by the high overall fish density, especially for smallmouth bass. Diet composition, studied in 2018, overlapped significantly between the three species in riffle habitat and overlap became less significant in run and pool habitat where smallmouth bass fed increasingly on fish prey. We therefore suggest that the interactions between smallmouth bass and dace and chub shift from predominantly predation in slow flowing habitat to resource competition in faster flowing habitat.
Spawning of invasive smallmouth bass (Micropterus dolomieu) was studied in 2020 and 2021, in Lake Nojiri, Nagano, Japan. Male smallmouth bass began nest construction once water temperatures reached ...14°C, and eggs progressed to fry dispersal in approximately 2 weeks. The main spawning season was in late May through early June, with a smaller second peak of nests in late June through early July. Nests were constructed primarily under dock cover, and nest size and depth were positively correlated with male body size. Of the nests that received eggs, nearly all progressed successfully to fry dispersal. Time lapse videos of male guarding behavior revealed that as nests progressed, males rotated less and were suspended off the bottom substrate. Similarly, the presence of egg predators was determined by experimental removal of guarding males. Freshwater gobies (Rhinogobius kurodai) were the only egg predators observed and arrived on all nests in the egg stage. However, only a handful of gobies visited each nest, the total number of eggs predated was low, and eggs or alevins in the progressed stage were rarely predated. These results indicate that in Lake Nojiri, smallmouth bass spawning is highly successful and egg predators appear to have minimal effect on egg survival. The stable smallmouth bass fishery without stocking at this lake is likely a product of these favorable spawning conditions. The lack of native egg predators may also be a driving factor in the continued spread of smallmouth bass throughout Japan.
Invasive smallmouth bass spawning was studied in Lake Nojiri, Japan. Spawning was highly successful and nest predators were rare with minimal impact on egg survival.
Coexistence of species requires equalizing mechanisms that minimize fitness differences, which are balanced by stabilizing mechanisms that enhance negative intraspecific interactions versus ...interspecific ones. Here, we develop a simple theoretical framework that allows measuring the relative strength of intraspecific versus interspecific competition in dominance hierarchies. We use it to evaluate mechanisms promoting coexistence between two congeneric charr that compete for foraging positions, which strongly influence density‐dependent growth and survival.
Agonistic interactions (n = 761) among 71 Dolly Varden Salvelinus malma and whitespotted charr Salvelinus leucomaenis were measured by snorkelling in two pools in the sympatric zone of a Hokkaido stream during two summers. Interspecific dominance hierarchies, analysed using three methods, were closely correlated with fish length but the species treated each other equally. Ranks for the most dominant fish in each pool, determined directly by knockout experiments, were also virtually identical to ranks by length.
Similarly, exponential random graph modelling of the social networks provided no evidence that either species was dominant over the other. Instead, larger fish were more likely to win contests, especially over fish of the next lower ranks.
These results demonstrated that the two species were nearly ecological equivalents in accessing key resources in this sympatric zone. Nearly identical growth and stable densities over 4 years further supported this inference, although Dolly Varden were a minority (29% of the assemblage), a sign of some fitness difference.
Detailed foraging observations coupled with two concurrent studies revealed an effective stabilizing mechanism. Dolly Varden shifted to feeding directly from the benthos when drifting invertebrates declined, a behaviour enhanced by morphological character displacement, thereby partitioning food resources and enhancing intraspecific competition while avoiding agonistic encounters with whitespotted charr.
The plurality of evidence indicates that fitness differences between these ecologically equivalent species are small in this local assemblage, and balanced by resource partitioning, a modest stabilizing mechanism that promotes coexistence. The theoretical framework presented here is a useful tool to evaluate the strength of interspecific versus intraspecific competition, which combined with information on trade‐offs in ecological performance can contribute to a mechanistic understanding of species coexistence.
要旨
1. 複数種の共存には適応度の種間差を最小にする均等化機構が必要だが、これは負の種間相互作用と比べて負の種内相互作用を相対的に強める安定化機構と相補的に働く．我々は種間順位制における種内競争と種間競争の相対的強度を測定する理論的な枠組みを構築し，北海道の河川に同所的に生息するイワナ属２種 (オショロコマ Salvelinus malmaおよびアメマス S. leucomaenis) に適用した．両種は流下する水生昆虫類を摂食するための定位場所をめぐって争い，密度依存的な成長率と生存率を示すことが知られている.
これら２種のイワナが同所的に生息する北海道幌尻沢の２つの淵において、２年にわたりシュノーケリングによる潜水観察を行い，両種合計71個体による761回の攻撃行動を記録した．3つの手法で推定した個体の順位はいずれも魚種には影響を受けず，ほぼ体サイズによって決まっていた．この結果は各淵における最優位個体の除去実験によっても支持された.
同様に指数的ランダムグラフモデル (exponential random graph modeling) によるソーシャルネットワーク分析でも、一方の魚種が優位という結果は得られなかった．一方、体サイズで上回る上位の個体は、次の順位の個体を頻繁に攻撃する傾向が認められた.
これらの結果は、同所的空間スケールでの餌資源をめぐる争いにおいて両種が生態的に同等な関係にあることを示す．同じ場所での４年にわたる調査でも、両種の生息密度は比較的安定で成長速度も類似しており、この事実もこの推論を支持する．ただし，混生域のオショロコマ個体数の比率は29 %とアメマスよりも少ないことは適応度に種間差があることを意味するかもしれない.
同時に行った形態計測と摂食行動の詳細な観察によって、両種の適応度を均衡させるメカニズムも明らかになった．つまりオショロコマは下顎形態を変化させる適応（形質置換）により，流下昆虫の減少期には底生動物を直接ついばむ摂食スタイルに容易にシフトし，アメマスとの直接的な攻撃を回避しつつ餌資源を効率的に利用していた.
これらの複数の証拠は、局所的な空間スケールにおいて生態的に同等な２種の適応度の差はわずかで，この均等化機構と資源分割が生む適度な安定化機構とが相まって共存が促進されていることを示している．この研究で提唱した理論的枠組みは，種間競争と種内競争の相対的な強さを評価する際に有用であり，生態学的なトレードオフに関する情報と併せることで複数種の共存メカニズム解明がさらに進むことが期待される.
The authors develop a new theoretical framework for measuring relative strength of intraspecific versus interspecific competition in interspecific dominance hierarchies and apply it to free‐ranging stream charr in Hokkaido, Japan. They show that in a zone of sympatry the two species are nearly equivalent competitors, and that behavioural and morphological adaptations allow one species to shift to alternate prey when resources decline, providing a stabilizing mechanism that promotes species coexistence.
Abstract
The area around the Sea of Okhotsk and the Sea of Japan in the north-western North Pacific provides opportunities for studying the impact of climate oscillations on the evolutionary history ...of cold-temperate species independent of direct ice sheet development. We conducted a range-wide collection survey of white-spotted charr, Salvelinus leucomaenis, and integrated nuclear DNA and mitochondrial DNA (mtDNA) data to reveal the evolutionary history of this species. Phylogeographic analyses using genome-wide SNP data revealed the presence of five geographically distinct groups with slightly overlapping ranges. Of these, a northern group was widely distributed along the coast of the Sea of Okhotsk, the Sea of Japan and the northern region of Honshu Island, whereas the other four groups were only found on Honshu Island. The population structure of Honshu Island was largely divided into the Sea of Japan and Pacific Ocean groups by the mountain ranges running through the centre of Honshu Island, and was further subdivided by many geological factors, such as river capture and other vicariance processes. There was a discrepancy in the phylogeographic patterns between nuclear DNA and mtDNA, which was likely influenced by a complex population history that involved multiple dispersal and secondary contact events during several glacial-interglacial cycles.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Foraging behavior and microhabitat use of native Japanese dace, pale chub, and invasive smallmouth bass were studied in the Nogu River, Nagano, Japan. Chub, dace and smallmouth bass all used drift ...feeding as their dominant foraging behavior. Chub drift fed most frequently, followed by dace, and smallmouth bass drift fed significantly less frequently than both native species. Chub and dace utilized microhabitat that had significantly higher flow velocity than smallmouth bass. However, smaller chub and dace were found in areas with lower flow velocity compared to larger individuals. Prey resource competition between smallmouth bass and native cyprinids, especially smaller individuals, is suspected due to similar feeding methods and microhabitat use.
Spawning season and habitat of invasive smallmouth bass were studied for two seasons in the Chikuma River, Nagano, Japan. Smallmouth bass spawning started in early May, when required water ...temperatures were met, and continued as long as spawning was not disrupted by high water levels. Spawning sites were found in patches throughout the study area that had significantly different physical characteristics than typical stream habitat features. Utilized spawning sites had low current velocity, were relatively shallow, and had pebble-sized substrate. Individual nest variables showed correlation with nesting male size as larger males constructed larger and deeper nests in faster flowing and deeper areas of the river. Nest guarding rates were low while egg presence rates were high compared to the native range, which may indicate that factors such as favorable water levels and water temperature and prevalence of spawning habitat facilitate spawning and the spread of smallmouth bass in the Chikuma River.
Abstract
Similar species that overlap in sympatry may diverge in characters related to resource use as a result of evolution or phenotypic plasticity. Dolly Varden charr (Salvelinus malma) and ...whitespotted charr (S. leucomaenis) overlap along streams in Hokkaido, Japan, and compete by interference for invertebrate drift-foraging positions. Previous research has shown that as drift declines during summer, Dolly Varden shift foraging modes to capture benthic prey, a behaviour facilitated by their subterminal jaw morphology. We compare body and jaw morphology of Dolly Varden in sympatry vs. allopatry in two locations to test for character displacement. Statistical analysis showed significant divergence in characters related to foraging, which was correlated with variation in individual charr diets. Dolly Varden in sympatry had shorter heads and lower jaws than in allopatry, and even within sites charr with these characteristics fed less on drifting terrestrial invertebrates but more on benthic aquatic invertebrates. Those in allopatry had longer heads and lower jaws, and fed more on terrestrial invertebrates. The close proximity of sites in one stream suggests that Dolly Varden may display phenotypic plasticity similar to other charr, allowing rapid responses in morphology to the presence of competitors. These morphological shifts probably help them maintain positive fitness when competing with whitespotted charr in Hokkaido streams.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
In Lake Nojiri, the Japanese smelt,
Hypomesus nipponensis
McAllister, 1963, population has been thought to be maintained by artificial spawning, and stocking efforts as natural spawning in in-flowing ...streams is unlikely due to the lack of inflowing stream habitat. In this study, novel resident Japanese smelt spawning was observed along the lakeshore in shallow areas with clean gravel and flow. Spawning occurred at night during early March to mid-April and eggs, confirmed on gravel substrate, progressed to the eyed state in 5–6 weeks. Although natural spawning was observed, the ratio of eggs that progressed to the eyed stage was minimal, possibly due to thick algae cover in areas with low flow velocity. Therefore, resident Japanese smelt spawning along the lake shoreline is physically possible and potentially contributes to the overall population in Lake Nojiri, but in-lake spawning alone presumably is not a biologically viable method for population sustainability due to low survival rates and egg density at present. Environmental improvements such as construction of fish ladders to suitable in-flowing spawning habitat, introduction of clean gravel to shorelines, and water level management adjusted to the smelt spawning run would contribute to higher recruitment by natural smelt reproduction, and consequently enhance the smelt production in Lake Nojiri.
