Seasonal diet shifts and trophic position of an invasive cyprinid, the rudd Scardinius erythrophthalmus (Linnaeus, 1758), in the upper Niagara River

Abstract

Cladophora is a genus of branched filamentous green algae (Ulvophyceae). It contains many species that are challenging to differentiate based on morphology because of the scarcity of diagnostic characters and extensive phenotypic plasticity. Within the past five years, Cladophora blooms have been observed on the ropes of green-lipped mussel farms in the Marlborough Sounds, New Zealand. When Cladophora reaches high biomass, it can clog mussel-harvesting equipment; thus, it is considered a nuisance organism in the region. This study used morphological and molecular techniques to identify the species responsible for the blooms, and to investigate whether this might be a recent incursion. Cladophora samples (n = 21) were collected from nine mussel farms, one salmon farm, and a marina. Morphological and phylogenetic analyses (partial large subunit and internally transcribed spacer regions 1 and 2 of the nuclear ribosomal cistron), revealed the identity of the bloom forming species as Cladophora ruchingeri (C.Agardh) Kützing, 1845. This represents the first report of this species in the Southern Hemisphere and Pacific region. Given the distinct morphology of C. ruchingeri (when mature), its absence from previous surveys of macro-algae from this region, and increasing reports of blooms, our findings suggest that this species has only recently been introduced to New Zealand. This study provides a robust taxonomic identification and initial baseline data. Further directed studies on Cladophora are required to advance knowledge on its ecology and distribution in New Zealand, and assist in the development of mitigation strategies.

Distribution of Herbivorous Fish Is Frozen by Low Temperature

“…The observed tendency towards switching between food sources is consistent with other studies on rudd. Based on our results, temperatures approximately 20 °C seem to be a crucial threshold for transitioning to herbivory263031. This trend is not only valid in our chosen model organism but can also be observed in other species.…”

“…Therefore, the cellulase produced by microorganisms capable of living in the digestive tracts of ectotherms can function at lower temperatures in comparison to other water organisms, where optimum temperatures for cleaving complex carbohydrates range from 30–60 °C444546. Although the lower limit for cellulose functioning in microorganisms is 15 °C, herbivory by fish still seems to be unbeneficial from 16–19 °C, (based on our observation263032), probably due to low cellulase activity. Therefore, the precise temperature at which plant diet starts to be ignored by fish will vary by species and individual.…”

“…However, the key environmental factor driving these preferences is still unknown. According to previous findings, temperature seems to play an important role, as it is positively correlated with the intake of plant matter by omnivorous fish14263032. This trend has also been observed in other fish species33.…”

“…These results are consistent with our findings that rudd in both lakes utilized only animal prey in the spring, when the surface water temperature ranged from 13.1 to 14.2 °C. Piscivory was even observed in rudd during the spring, in low temperatures in the Niagara River26.…”

“…The proportion of plant-based diet consumed (mainly macrophytes) is markedly higher for rudd than for other species, e.g., roach ( Rutilus rutilus ), even when a nutrient-rich diet is available26. Recent studies examining macrophyte characteristics and the causes of selective herbivory in rudd assert that the greatest factors are C:N ratio and phenolic concentration, both of which are negatively correlated with herbivory rates.…”