Prey selection by harbour seals, Phoca vitulina, in relation to variations in prey abundance

1997 ◽  
Vol 75 (9) ◽  
pp. 1508-1518 ◽  
Author(s):  
Dominic J. Tollit ◽  
Paul M. Thompson ◽  
Simon P. R. Greenstreet
Keyword(s):  
Diet Composition ◽  
Prey Size ◽  
Prey Selection ◽  
Phoca Vitulina ◽  
Correction Factors ◽  
Fish Prey ◽  
Size Selection ◽  
Harbour Seals ◽  
Moray Firth ◽  
Time Periods

This study compared the composition of the diet of harbour seals (Phoca vitulina) feeding in the Moray Firth, Scotland, with the abundance of their fish prey estimated from dedicated fishery surveys in January 1992 and 1994 and June 1992. Prey-size selection was also examined in these three time periods and in January 1991. In each period, the most abundant fish species contributed most to the diet. However, the relative abundances of the remaining species in the sea showed little similarity to their contribution to the seals' diet. Diet composition was almost totally dominated by either pelagic species or species dwelling on or strongly associated with the seabed, depending upon the relative abundance of pelagic schooling prey. Most fish consumed were 10–16 cm in length, although larger cod and herring were taken. With the exception of cod, the extent of size selection was dependent upon the use of correction factors that accounted for otolith erosion due to digestion.

Species and size differences in the digestion of otoliths and beaks: implications for estimates of pinniped diet composition

1997 ◽  
Vol 54 (1) ◽  
pp. 105-119 ◽  
Author(s):  
D J Tollit ◽  
M J Steward ◽  
P M Thompson ◽  
G J Pierce ◽  
M B Santos ◽  
...  
Keyword(s):  
Confidence Intervals ◽  
North Sea ◽  
Diet Composition ◽  
Prey Size ◽  
Phoca Vitulina ◽  
Correction Factors ◽  
Fish Size ◽  
Harbour Seals ◽  
Specific Correction ◽  
Length Reduction

We examined the digestion of hard remains of between one and four different size ranges of nine key North Sea prey taxa fed to seven captive harbour seals (Phoca vitulina). Percentage length reduction (mean 27.5%) and recovery rates (mean 42%) of experimental otoliths varied between species and were positively correlated to fish size and otolith robustness (mass/length). Mean length reduction of egested otoliths increased systematically with increasing size of ingested whiting and sandeel otoliths (p < 0.001), indicating that the size of larger fish may be underestimated. Intraspecific variation in otolith digestion was high (CV = 0.48-1.30), and to control for the artificial conditions of a captive study, external morphological features of otoliths were used to grade the degree of digestion and provide grade-specific correction factors. Bootstrap simulations were used to estimate 95% confidence intervals around correction factors and when partitioned indicated that calculation errors were in general less important than resampling errors. The application of species-, size-, and grade-specific correction factors progressively improved reconstructed estimates of prey biomass fed. As a consequence, estimates of prey size and diet composition require otoliths from faeces to be graded and more complex correction factors applied.

scholarly journals Fish size selection and diet composition of Eurasian otters (Lutra lutra) in salmonid streams: Picky gourmets rather than opportunists?

2019 ◽  
pp. 29 ◽  
Author(s):  
Marcia Sittenthaler ◽  
Lucia Koskoff ◽  
Kurt Pinter ◽  
Ursula Nopp-Mayr ◽  
Rosemarie Parz-Gollner ◽  
...  
Keyword(s):  
Diet Composition ◽  
Multiple Stressors ◽  
Prey Selection ◽  
Temporal Variations ◽  
Salmonid Fish ◽  
Habitat Characteristics ◽  
Lutra Lutra ◽  
Fish Prey ◽  
Dietary Analysis ◽  
Size Selection

Knowledge on predator diet and drivers of prey selection is particularly of interest for an efficient management of predator and prey populations where predators potentially compete with humans for resources. Actual or perceived predation by Eurasian otter (Lutra lutra) on fish stocks generates conflicts in many countries. Recently, conflicts are heating up in riverine habitats, where multiple stressors affect stream fish populations. We combined dietary analysis of otter faeces and prey fish availability in three Austrian streams to assess spatial and seasonal differences in diet composition, the extent of (salmonid) fish consumption and the selection for specific salmonid fish sizes relative to their availability. Otters in upper reaches of temperate salmonid streams occupied a narrow trophic niche. Overall, otters fed predominantly on fish with salmonids dominating diet, both in terms of frequency and ingested biomass measures. Within the category of salmonids, otters selected for specific size classes. Concurrently, otters also displayed an opportunistic feeding behaviour, and seasonally and locally non-fish prey and other fish species than salmonids became key resources. Diet composition and salmonid size selection varied significantly within and between streams, which we relate to spatio-temporal variations of prey community composition and stream-specific habitat characteristics affecting prey vulnerability.

Seasonal and between-year variations in the diet of harbour seals in the Moray Firth, Scotland

1996 ◽  
Vol 74 (6) ◽  
pp. 1110-1121 ◽  
Author(s):  
D. J. Tollit ◽  
P. M. Thompson
Keyword(s):  
Diet Composition ◽  
Gadus Morhua ◽  
Prey Species ◽  
Temporal Variations ◽  
Short Term ◽  
Harbour Seals ◽  
Faecal Samples ◽  
Poor Indicator ◽  
Moray Firth ◽  
Merlangius Merlangus

Predictions concerning the effect of seals upon prey stocks require an understanding of temporal variations in diet composition. This study examined the extent of between-year and seasonal variations in the diet of harbour seals (Phoca vitulina) from the Moray Firth, Scotland, between 1989 and 1992. Analyses of fish otoliths and cephalopod beaks collected from 1129 faecal samples were used to derive estimates of the contribution made by each of 35 prey species, based on the number and mass consumed. The key prey, by mass, were sand eels (Ammodytidae) (47%), lesser octopus (Eledone cirrhosa) (27%), whiting (Merlangius merlangus) (6%), flounder (Platichthys flesus) (5%), and cod (Gadus morhua) (4%). Between-year and seasonal fluctuations in the contributions of these species were observed. Sand eels contributed 86–20% in summer and 91–49% in winter. Lesser octopus contributed 0–62% in summer and < 5% in winter, whilst whiting and cod contributed 2–34% in winter and 1–4% in summer. In contrast to 1988, clupeids were unimportant in winter. Differences in diet composition appeared to reflect local changes in the availability of food, especially overwintering clupeids. Results indicate that dietary information obtained from short-term studies can be a poor indicator of subsequent diet composition and should be treated with caution when used to predict the effect of seals on prey populations.

Frequency-dependent prey selection by larvae of Toxorhynchites splendens (Diptera: Culicidae)

1996 ◽  
Vol 86 (6) ◽  
pp. 633-639 ◽  
Author(s):  
D. Dominic Amalraj ◽  
P. K. Das
Keyword(s):  
Aedes Aegypti ◽  
Foraging Behaviour ◽  
Anopheles Stephensi ◽  
Prey Size ◽  
Prey Selection ◽  
Fourth Instar ◽  
Frequency Dependent ◽  
Size Selection ◽  
The Stability

AbstractThe foraging behaviour of frequency-dependent prey selection by larval instars of Toxorhynchites splendens (Wiedemann) was studied in the laboratory. Prey size selection (second vs fourth instars of Aedes aegypti Linnaeus or Anopheles stephensi Liston) by third and fourth instar predators was frequency-dependent. However, in the case of second instar predators, prey size selection was not frequency-dependent and the predator preferred second instar to fourth instar prey. When offered second instars of Aedes aegypti and Anopheles stephensi the preference for one species over the other was frequency-dependent in all the three predator instars. The role of frequency-dependent prey selection in the stability of prey—predator interaction at low equilibrium levels is discussed.

Diet of harbour seals at Mousa, Shetland, during the third quarter of 1994

1997 ◽  
Vol 77 (2) ◽  
pp. 539-555 ◽  
Author(s):  
E.G. Brown ◽  
G.J. Pierce
Keyword(s):  
Prey Availability ◽  
Temporal Trends ◽  
Phoca Vitulina ◽  
Correction Factors ◽  
Fish Size ◽  
The Third ◽  
Feeding Experiments ◽  
Harbour Seals ◽  
Trawl Catches ◽  
Fish Otoliths

Scats (N=200) of harbour seals (Phoca vitulina) were collected from haul-out sites on the Island of Mousa, south-east Shetland, during July, August and September 1994. A total of 5,531 fish otoliths were identified in scats and measured to reconstruct fish lengths and weight. Gadid fishes dominated the diet of harbour seals on Mousa during the study period, both in terms of weight and the frequency with which they occurred. There was evidence for the existence of temporal trends. Over the period July–September the importance of gadids in the diet increased while that of sandeels declined. These temporal trends are probably associated with changes in prey availability. Comparisons of the size of sandeels, herring and whiting eaten by seals with trawl catches suggest that seals were probably not size-selective, but a corollary of this is that, while otolith size is reduced during passage through seal guts, application of correction factors derived from captive feeding experiments led to over-estimates of fish size.

The effect of weather conditions on harbour seal (Phoca vitulina) haulout behaviour in the Moray Firth, northeast Scotland

1996 ◽  
Vol 74 (10) ◽  
pp. 1806-1811 ◽  
Author(s):  
Kate Grellier ◽  
Paul M. Thompson ◽  
Heather M. Corpe
Keyword(s):  
Significant Relationship ◽  
Seasonal Changes ◽  
Negative Relationship ◽  
Weather Conditions ◽  
Phoca Vitulina ◽  
Effect Of Temperature ◽  
Harbour Seals ◽  
Moray Firth ◽  
A Site ◽  
The Relationship

Abundance data from a 6-year study (1988–1993) of harbour seals (Phoca vitulina) were used to control for seasonal changes in haulout behaviour and assess the effect of temperature and other weather conditions on haulout numbers at a site in Scotland. A significant relationship between Julian day and haulout numbers was consistent between years. In some years, there was also a significant relationship between ambient temperature and haulout numbers, but examination of the residuals around the relationship between haulout numbers and Julian day revealed no evidence for a consistent effect of temperature, wind speed, or wind-chill adjusted temperatures. A weak negative relationship between haulout numbers and both precipitation and cloud cover did exist, but these variables explained little of the variation in seal numbers.

Evaluating fish diet analysis methods by individual-based modelling

2012 ◽  
Vol 69 (7) ◽  
pp. 1184-1201 ◽  
Author(s):  
Ida Ahlbeck ◽  
Sture Hansson ◽  
Olle Hjerne
Keyword(s):  
Diet Composition ◽  
Prey Size ◽  
Diet Analysis ◽  
Fish Diet ◽  
Relative Importance ◽  
Fish Prey ◽  
Analysis Methods ◽  
Composite Indices ◽  
Index Of Relative Importance ◽  
Better Than

Knowledge of diet compositions is important in ecological research. There are many methods available and numerous aspects of diet composition. Here we used modelling to evaluate how well different diet analysis methods describe the “true” diet of fish, expressed in mass percentages. The methods studied were both basic methods (frequency of occurrence, dominance, numeric, mass, points) and composite indices (Index of Relative Importance, Comparative Feeding Index). Analyses were based on both averaged stomach content of individual fish and on pooled content from several fish. Prey preference, prey size, and evacuation rate influenced the performance of the diet analysis methods. The basic methods performed better than composite indices. Mass and points methods produced diet compositions most similar to the true diet and were also most robust, indicating that these methods should be used to describe energetic–nutritional sources of fish.

The influence of body size and sex on the characteristics of harbour seal foraging trips

1998 ◽  
Vol 76 (6) ◽  
pp. 1044-1053 ◽  
Author(s):  
Paul M Thompson ◽  
Ann Mackay ◽  
Dominic J Tollit ◽  
Simon Enderby ◽  
Philip S Hammond
Keyword(s):  
Body Size ◽  
Activity Pattern ◽  
Diet Composition ◽  
Phoca Vitulina ◽  
Trip Duration ◽  
Energy Requirements ◽  
Endogenous Factors ◽  
Foraging Range ◽  
Harbour Seals ◽  
Size Population

Most pinnipeds disperse from centralised terrestrial sites to forage at sea, but the factors that result in variation inforaging-trip characteristics remain unclear. We investigated the influence of sex and body size on the summer foraging activityof radio-tagged harbour seals (Phoca vitulina) from Scotland. Mean foraging-trip duration (range 17–257 h) was stronglycorrelated with mean foraging range (range 4.3–55.0 km), but both were significantly shorter for females. The proportion oftime spent at sea, mean trip duration, and mean foraging range were all positively related to body size. Comparison with datafrom other study areas suggests that both environmental and endogenous factors shape foraging characteristics in this species.These sex and body size related differences in activity pattern and foraging range have important implications for themethodologies currently used to assess the population size, population energy requirements, and diet composition of coastalpinnipeds.

Explaining diet composition of North Sea cod (Gadus morhua): prey size preference vs. prey availability

2003 ◽  
Vol 60 (2) ◽  
pp. 140-150 ◽  
Author(s):  
Jens Floeter ◽  
Axel Temming
Keyword(s):  
North Sea ◽  
Diet Composition ◽  
Gadus Morhua ◽  
Prey Size ◽  
Prey Availability ◽  
Weight Ratio ◽  
Fish Prey ◽  
Gear Efficiency ◽  
Species Specific ◽  
Size Preference

The nature and significance of size preference for fish prey in the diet selection of North Sea cod (Gadus morhua L.) was analysed. The analysis combined information on size-specific abundance derived from bottom trawl surveys with prey size frequencies in cod stomachs from the International North Sea Stomach Database. To estimate the abundance of all potential fish prey in the sea, a length-based number spectrum was calculated, corrected for gear efficiency by the application of a species-specific correction factor, and weighted by local predator abundance to take the spatial–temporal overlap between cod and its prey into consideration. A prey size preference model for cod feeding on fish is presented. Results showed that the preferred predator-to-prey weight ratio is an exponentially increasing function of predator size and an exponentially decreasing function of the slope of the number spectrum. More than 75% of fish found in the stomachs of North Sea cod originated from the least preferred quartile of the prey size range, indicating that prey abundance is the main determinant of the diet composition.

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