Feeding Aquatic Ecosystems: Whole-Lake Experimental Addition of Angler’s Ground Bait Strongly Affects Omnivorous Fish Despite Low Contribution to Lake Carbon Budget

Effects of angler's groundbaits on fish physiology and growth

Although angler's groundbaits (GBs) can be an important food resource for fish, we do not know much about the effects of GB consumption on the growth and health of fish. To fill this knowledge gap, we conducted a controlled, six-week long feeding trial (feeding ration: 2% of body weight) with common carp at 22 °C (Cyprinus carpio, mean initial body weight: 557 g) to test the effect of two GBs composed mostly of animal-derived ingredients (AN-GBs) and two plant-based GBs (PL-GBs) relative to one aquaculture feed, as a control (five treatment altogether). Consumption of PL-GBs resulted in lower growth rate than AN-GBs, presumably due to the low protein content. However, the unit biomass increment per unit nitrogen input was higher in PL-GBs. Although PL-GBs resulted in reduction of hepatic energy reserves, hepatosomatic index, viscerosomatic index, and body condition did not differ among the treatments. We did not find differences in expression of inflammatory cytokines in the liver. In conclusion, AN-GBs more effectively increases the carrying capacity of fisheries, but fish sequester a higher portion of nitrogen content of PL-GBs -PL-GB input can be more effectively counterbalanced by fish removal. Finally, the GB consumption does not pose a health risk to fish.

Acute effects of angler's groundbaits: nutrient flux to water column

Although ground-baiting related nutrient loading has been widely studied, we do not know what proportion of these nutrients release into the water column, affecting primary production directly. We conducted short-term (24-h, 5-day) experiments at wide temperature range, in presence and absence of fish using fish meal-based (FM-GB) and plant-based groundbait (PB-GB), to assess the nitrogen (N) and phosphorus (P) fluxes from GB into the water column. Nitrogen release from unconsumed FM-GB was negligible in the first 3 days, then increased abruptly, releasing 32% of its total N content by the fifth day. In contrast, PB-GB acted as temporary sink for inorganic N forms. Considerable (18-21%) inorganic P release was observed in both GB types in the first twelve hours. Consumed GBs induced considerable inorganic N release and its rate increased with temperature. Particulate forms predominated the released N in PB-GB, suggesting impaired digestion. Phosphorus-dominated by particulate forms-release was similar or lower than in unconsumed GB. Based on our results, excessive use of GB-when high amount of it remains unconsumed-can enhance eutrophication in P-limited ecosystems. Although less digestible GBs may have less abrupt effect on the primary production, undigested nutrients remain unavailable for removal through fish harvest.

Rhythm of relationships in a social fish over the course of a full year in the wild

BACKGROUND

Animals are expected to adjust their social behaviour to cope with challenges in their environment. Therefore, for fish populations in temperate regions with seasonal and daily environmental oscillations, characteristic rhythms of social relationships should be pronounced. To date, most research concerning fish social networks and biorhythms has occurred in artificial laboratory environments or over confined temporal scales of days to weeks. Little is known about the social networks of wild, freely roaming fish, including how seasonal and diurnal rhythms modulate social networks over the course of a full year. The advent of high-resolution acoustic telemetry enables us to quantify detailed social interactions in the wild over time-scales sufficient to examine seasonal rhythms at whole-ecosystems scales. Our objective was to explore the rhythms of social interactions in a social fish population at various time-scales over one full year in the wild by examining high-resolution snapshots of a dynamic social network.

METHODS

To that end, we tracked the behaviour of 36 adult common carp, Cyprinus carpio, in a 25 ha lake and constructed temporal social networks among individuals across various time-scales, where social interactions were defined by proximity. We compared the network structure to a temporally shuffled null model to examine the importance of social attraction, and checked for persistent characteristic groups over time.

RESULTS

The clustering within the carp social network tended to be more pronounced during daytime than nighttime throughout the year. Social attraction, particularly during daytime, was a key driver for interactions. Shoaling behavior substantially increased during daytime in the wintertime, whereas in summer carp interacted less frequently, but the interaction duration increased. Therefore, smaller, characteristic groups were more common in the summer months and during nighttime, where the social memory of carp lasted up to two weeks.

CONCLUSIONS

We conclude that social relationships of carp change diurnally and seasonally. These patterns were likely driven by predator avoidance, seasonal shifts in lake temperature, visibility, forage availability and the presence of anoxic zones. The techniques we employed can be applied generally to high-resolution biotelemetry data to reveal social structures across other fish species at ecologically realistic scales.

The body condition of invasive crayfish Faxonius limosus (Raf., 1817) (Decapoda: Cambaridae) is better in small rivers than in dam reservoirs in Central Europe

The impact of dam reservoirs on river ecosystems is one of the current major issues in hydrobiology. In addition to the impact on native biodiversity, impoundments may facilitate biological invasions. A successful invasion is presumably related to the individual life history and resource allocation strategy in expanding populations. We tested whether reservoirs affect the body condition of invasive Faxonius limosus in upland streams. We found positive allometry of growth in females but more frequent isometric growth in males, and the form factor a_3.0 was significantly higher in males than in females within populations. Fulton's condition factor (K) was calculated for standard carapace lengths of 20 mm (juvenile) and 40 mm (adult). Both mean K_{CL 20} and K_{CL 40} were higher in males than in females. Positive allometry of growth in females resulted in a larger mean K_{CL 40} than K_{CL 20}. The majority of the mean K obtained in reservoirs was similar or lower than the respective condition in streams. The condition of adult females was higher in streams in comparison to reservoirs. The obtained results are in contrast to the hypothesis that reservoirs positively affect individual body conditions and show that these man-made lakes do not provide more favourable habitat conditions for F. limosus than those in the natural aquatic environments of an upland landscape. An advantageous effect of the presence of reservoirs on invasion spread may be possible in more ecologically harsh landscapes, especially in colder climatic zones. Our study is the first to investigate the relationship of the body condition of F. limosus in two contrasting habitats.

Big-data approaches lead to an increased understanding of the ecology of animal movement

Understanding animal movement is essential to elucidate how animals interact, survive, and thrive in a changing world. Recent technological advances in data collection and management have transformed our understanding of animal "movement ecology" (the integrated study of organismal movement), creating a big-data discipline that benefits from rapid, cost-effective generation of large amounts of data on movements of animals in the wild. These high-throughput wildlife tracking systems now allow more thorough investigation of variation among individuals and species across space and time, the nature of biological interactions, and behavioral responses to the environment. Movement ecology is rapidly expanding scientific frontiers through large interdisciplinary and collaborative frameworks, providing improved opportunities for conservation and insights into the movements of wild animals, and their causes and consequences.

Network analysis of intra- and interspecific freshwater fish interactions using year-around tracking

A long-term, yet detailed view into the social patterns of aquatic animals has been elusive. With advances in reality mining tracking technologies, a proximity-based social network (PBSN) can capture detailed spatio-temporal underwater interactions. We collected and analysed a large dataset of 108 freshwater fish from four species, tracked every few seconds over 1 year in their natural environment. We calculated the clustering coefficient of minute-by-minute PBSNs to measure social interactions, which can happen among fish sharing resources or habitat preferences (positive/neutral interactions) or in predator and prey during foraging interactions (agonistic interactions). A statistically significant coefficient compared to an equivalent random network suggests interactions, while a significant aggregated clustering across PBSNs indicates prolonged, purposeful social behaviour. Carp (Cyprinus carpio) displayed within- and among-species interactions, especially during the day and in the winter, while tench (Tinca tinca) and catfish (Silurus glanis) were solitary. Perch (Perca fluviatilis) did not exhibit significant social behaviour (except in autumn) despite being usually described as a predator using social facilitation to increase prey intake. Our work illustrates how methods for building a PBSN can affect the network's structure and highlights challenges (e.g. missing signals, different burst frequencies) in deriving a PBSN from reality mining technologies.