DOI: 10.5281/zenodo.15874032
title: Great cormorants and grey herons depredating at finfish aquaculture: Factors affecting the human–wildlife conflict

description: Sustainable aquaculture requires consideration of its interactions with wildlife. Human-wildlife conflicts arise when piscivorous birds, such as cormorants and herons, depredate in fish farms. By surveillance cameras, we quantified the depredation pressure of birds at fish farms along the Finnish coast. The pressure varied considerably between farms, from no bird visits to daily losses of hundreds of fish. Grey herons were most numerous and depredated 2–5 times more fish than cormorants, which are commonly regarded bigger threats. Depredation rates decreased with the increasing fish size even though cormorants also took large fish. Piscivorous raptors seldom foraged in farms but were interested in larger fish. Proximity to bird breeding colonies did not affect the predation pressure. Protective nets were effective against raptors and cormorants, but grey herons used them as depredation platforms. Knowledge on factors explaining depredation rates is valuable for science-based planning of measures to mitigate the human-wildlife conflict.
description: Data from the publication:

Great cormorants and grey herons depredating at finfish aquaculture: Factors affecting the human–wildlife conflict. Ambio (2025) https://doi.org/10.1007/s13280-025-02218-5

Description of the data

This data was obtained by analyzing video material from fish cages along the Finnish coast, collected in 2022-2023. We counted the occurrence and depredation by grey herons (Ardea cinerea), great cormorants (Phalacrocorax carbo sinensis), white-tailed eagles (Haliaeetus albicilla) and ospreys (Pandion haliaeetus). Due to the sensitivity of the spatial data, the scaled latitudinal parameter is rounded to two decimals. 

Cage_ID: Unique identifier for each fish cage. Three cages were monitored in both 2022 and 2023, these are indicated by 'a' (2022) and 'b' (2023).

Location: Municipality

Year: year

yymmdd: date (year, month, day)

Month.f: Month as categorical value. The two days analysed in late April were attributed to May (5).

Species



Grey heron (Ardea cinerea)

Raptor: White-tailed eagles and ospreys pooled to raptors

Cormorant: Great cormorant (Phalacrocorax carbo sinensis)


Occurrence: number of minutes during the analyzed day that birds of the species were spending at the cage (possibly foraging, i.e. herons standing on the net, cormorants swimmimg in the cage, and raptors flying close over)

Depredation_corrected: Number of fish seen caught by the birds during the day.  Herons were foraging at the cages also at some dark hours which could not be analyzed. To correct for this, in the days where not all hours could be analyzed, the number of fish was divided by the number of analyzed hours and multiplied with 24.

Depredation_orig: The actual number of fish seen caught by the birds. For cormorants and raptors depredation_corrected equals depredation_orig.

Attempts_corrected: Number of depredation attempts (for herons: a peck, for cormorants: a dive, for raptors: dipping the feet in the water). Herons were foraging at the cages also at some dark hours which could not be analyzed. To correct for this, in the days where not all hours could be analyzed, the number of attempts were divided by the number of analyzed hours and multiplied with 24.

Attempts_orig: The actual numbers of attempts seen in the video material. For cormorants and raptors attempts_corrected equals attempts_orig.

Analyzed_hours: Numbers of hours in the day that were possible to analyze.

Fish_size: Weight of the fish in the cage at that day in kg

Distance: The distance to the nearest colony of the particular bird in meters. Not applicable for raptors.

Distance_Ac: distance (m)to the nearest Grey heron colony

Distance_Pc: distance (m) to the nearest Great cormorant colony

zFish_size: Weight of the fish in the cage at that day (standardised)

zDistance_Ac: distance to the nearest Grey heron colony (standardised)

zDistance_Pc: distance to the nearest Great cormorant colony (standardised)

zLat: latitude (Euref89), standardised and rounded to two decimals

 
description: The study was funded by the Ministry of the Environment in Finland. Funding for the writing of the article was obtained from the European Maritime, Fisheries andAquaculture Fund (EMFAF).