Bird stomach contents analysis - final report: Goosander and Cormorant diet on four Scottish rivers 2019 to 2020

This study analysed the stomach contents of goosanders and cormorants collected from the Rivers Tweed, Dee, Nith and Spey during 2019 and 2020 in order to to assess whether there was evidence of substantial changes in the diets of these species of fish-eating birds since the 1990s.

Appendix 1: Stomach contents analysis and diet assessment methodology

Processing of bird carcasses and wet prey remains

Bird carcasses

1.1 Fit each carcass with a permanent label affixed to the leg to guarantee individual identity throughout processing.

1.2 Where necessary, thaw bird carcasses at room temperature for 24-48 hrs.

1.3 Identify birds to species and sex (and age where possible based on plumage characteristics).

1.4 Take beak measurements from Cormorants (to investigate possible racial differences).

1.5 Weigh birds (nearest 10g) and measure wing (nearest 1 mm).

1.6 Dissect birds from the beak to the vent to record all material in the digestive tract.

Prey remains: wet

1.7 Remove any whole ('freshly eaten') fish carefully, for identification and measurement.

1.8 Flush out all remaining partially-digested material and collect in individually labelled sample jars.

1.9 Digest these remains (4-6 days in oven at ca. 37˚C) in a saturated solution of biological washing powder to remove all flesh.

1.10 Rinse the digested samples thoroughly with cold water through 1.0mm and 0.5mm sieves until completely clean.

1.11 Transfer thoroughly-rinsed bones to filter paper and air dry (1-3 days).

1.12 Transfer dried material to individually labelled paper envelopes for dry analysis and/or storage.

Identification, counting and measuring of dry prey remains

Prey remains: dry

2.1 Transfer dried material to Petri dish for examination under low powered binocular microscope.

2.2 Ignore obvious highly-eroded (well digested) hard parts from previous meals.

2.3 Identify key bones to fish species and extract/store them (as described in literature and with extensive UK CEH reference collection).

2.4 Measure specific key bones (e.g. Salmonid atlas vertebrae, cyprinid pharyngeal teeth) to estimate original fish length.

2.5 Identify any other remains (e.g. some specific species/bones) and estimate fish lengths from comparison with curated UK CEH reference material.

Calculating proportions (mass, number) of each species in the diet

Subsequent calculations

3.1 Determine the numbers of individuals recorded for each fish species.

3.2 For each individual fish, convert each length estimate (or measurement in the case of 'fresh fish) into an estimate of body mass from a series of length:mass equations (from the published literature and some based on previous UK CEH work e.g. Appendix in Carss & Ekins 2002).

3.3 For each fish species, combine numbers and estimated individual mass to estimate the total (mass) contribution.

3.4 Combine the mass of each fish species to determine the overall mass of whole sample.

3.5 Derive the proportion (by mass and by number) of each fish species in the diet, collated in a form directly comparable (and equivalent) "as contained in appendix 5.1 and 5.3, for Goosanders and Cormorants respectively, of the 'Fish-eating Birds and Salmonids in Scotland' report published in November 1998 (ISBN 0 7480 7232 2)" (7.1 of contract requirements).



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