DOI: 10.5061/dryad.4j0zpc89q
title: Predator-prey overlap in three dimensions: cod benefit from capelin coming near the seafloor

description: Spatial overlap between predator and prey is a prerequisite for predation,
 but the degree of overlap is not necessarily proportional to prey
 consumption. This is because many of the behavioural processes that
 precede ingestion are non-linear and depend on local prey densities. In
 aquatic environments, predators and prey distribute not only across a
 surface, but also vertically in the water column, adding another dimension
 to the interaction. Integrating and simplifying behavioural processes
 across space and time can lead to systematic biases in our inference about
 interaction strength. To recognise situations when this may occur, we must
 first understand processes underlying variation in prey consumption by
 individuals. Here we analysed the diet of a major predator in the Barents
 Sea, the Atlantic cod (Gadus morhua), aiming to understand drivers of
 variation in cod’s feeding on its main prey capelin (Mallotus villosus).
 Cod and capelin only partly share habitats, as cod mainly reside near the
 seafloor and capelin inhabit the free water masses. We used data on
 stomach contents from ~2000 cod individuals and their surrounding
 environment collected over 12 years, testing hypotheses on biological and
 physical drivers of variation in cod’s consumption of capelin, using
 Generalized Additive Models. Specifically, effects of capelin abundance,
 capelin depth distribution, bottom depth, and cod abundance on capelin
 consumption were evaluated at a resolution scale of 2 km. We found no
 indication of food competition as cod abundance had no effect on capelin
 consumption. Capelin abundance had small effects on consumption, while
 capelin depth distribution was important. Cod fed more intensively on
 capelin when capelin came close to the seafloor, especially at shallow
 banks and bank edges. Spatial overlap as an indicator for interaction
 strength needs to be evaluated in three dimensions instead of the
 conventional two when species are partly separated in the water column.
description: The data was collected by
 Norwegian vessels participating in the Norwegian-Russian Barents Sea
 Ecosystem Survey in 2004-2015. The survey covers the entire ice-free
 Barents Sea in August-October each year and follows a regular grid design
 with sampling stations spaced approximately 65 km apart, collecting data
 on environmental conditions, species composition and abundance for several
 trophic levels. We selected stations from areas where cod and capelin
 overlap spatially (Fall, et al. 2018), i.e., stations were both species
 were observed. At each station, a CTD (Conductivity-Temperature-Depth)
 probe is lowered to measure depth-specific temperature, a Campelen 1800
 demersal shrimp trawl is used for near-bottom sampling over a distance of
 1.4 km (0.75 nautical miles, nmi), and a pelagic trawl (‘Harstad trawl’,
 Godø, et al. 1993) samples the upper approximately 60 m of the water
 column. Continuously during the survey, Simrad EK60 echo sounders with 18,
 38, 120 and 200 kHz split beam transducers (on some vessels Simrad EK500
 during the first years) record fish echoes along the survey tracks. The
 acoustic backscatter at 38 kHz is manually allocated to target groups
 based on species-specific acoustic signatures and the catch composition in
 pelagic and bottom trawls, then stored at a horizontal resolution of 1.9
 km (1 nmi) and a vertical resolution of 10 m. One individual cod from each
 5-cm length group present in the bottom trawl catch is randomly chosen for
 sampling of age, sex, mass, length, maturity stage, and stomach contents.
 The stomachs are frozen on board, and the contents identified to the
 lowest possible taxonomic level in a laboratory on land. Prey items are
 assigned a qualitative digestion stage from 1-5, where 1 corresponds to
 undigested prey and 5 to prey that is so heavily digested that it is
 unidentifiable by visual inspection. Prey categories are weighed (wet
 mass) and, if possible, counted and length measured. The dataset contains
 stomach data from cod in the size range 30-70 cm. Empty stomachs (9.2%)
 were excluded from the analyses, leaving 1944 stomachs from 455 stations
 across the 12 years. The following covariates are
 included in the dataset: capelin density, capelin weighted median depth,
 cod density, cod length (for each individual within the station), and
 bottom depth (measured by the vessel-mounted echosounder at the start of
 trawling). The local capelin density (in units of Nautical Area Scattering
 Coefficient; m<sup>2</sup>/nmi<sup>2</sup>,
 integrated throughout the water column) was taken from the acoustic
 segment of 1.9 km length that had the highest temporal overlap with each
 trawl haul. This included acoustic recordings from before, during and
 after trawling. For a more intuitive representation of prey density, we
 converted the acoustic values to number of
 individuals/km<sup>2</sup> based on the length distribution in
 the closest pelagic trawl hauls taken during the survey, and the
 relationship between capelin length and acoustic target strength
 (Gjøsæter, et al. 1998). For capelin weighted depth, we used the same
 acoustic segment resolved in 10 m depth channels to calculate the weighted
 median depth of the capelin backscatter at each trawl location. This depth
 was then standardised with respect to time of day at each trawl location
 by fitting a Generalised Additive model from which capelin depth was
 predicted for a fixed time of day (see <em>Standardisation of
 capelin vertical distribution</em> in the methods section of the
 article). The local cod density (number of individuals ≥ 30
 cm/km<sup>2</sup>) was estimated from each demersal trawl haul
 using standard methods for cod swept area calculation in the Barents Sea,
 which assume that the effective sweep width of the trawl increases with
 cod size up to 62 cm (Mehl, et al. 2014).  
description: <b>Details on the
 dataset and variables</b> <b>Date of data
 collection</b> 2004-08-16 --
 2015-09-18 <b>Geographic location of
 data collection</b> Barents Sea; longitude 18.41167
 -- 46.57667 E, latitude 72.33167 -- 80.10333 N (decimal degrees)
 <b>Number of
 variables</b> 42. Comma-separated.
 <b>Number of
 cases/rows</b> 1944. Each line represents a cod
 individual. <b>Variable
 list</b> Name: <b>year</b>. Description: The year the cod was sampled. Used as a covariate in the analysis. Name: <b>date</b>. Description: The date the cod was sampled. Name: <b>sampling.day</b>. Description: The day of the year the cod was sampled, in relation to the first day of the year that was sampled in the dataset (i.e., August 8<sup>th</sup> is day 0). Used as a covariate in the analysis. Name: <b>station.id</b>. Description: A unique ID for the sampling station. Several cod (lines in the dataset) may be sampled at each station. Used as a covariate in the analysis. Name: <b>start_time</b>. Description: start date and time for the trawl haul in which the cod was sampled. On the form year-month-day hour:min:sec. Name: <b>stop_time</b>. Description: stop date and time for the trawl haul in which the cod was sampled. On the form year-month-day hour:min:sec. Name: <b>lon</b>. Description: longitude of sampling station (decimal degrees). Name: <b>lat</b>. Description: latitude of sampling station (decimal degrees). Name: <b>x</b>. Description: easting of sampling station, expressed as distance in km from 35 degrees E (approximate center of the Barents Sea). Used as a covariate in the analysis. Name: <b>y</b>. Description: northing of sampling station, expressed as distance in km from 75 degrees N (approximate center of the Barents Sea). Used as a covariate in the analysis. Name: <b>bottomdepthstart</b>. Description: Bottom depth at the start of trawling. Name: <b>sunheight</b>. Description: Angle of the sun relative to the horizon (angle = 0) at the time and location of sampling. Calculated from the day of the year, geographical location, and time of day. Name: <b>sunrise</b>. Description: Angle of the sun at sunrise at the sampling location. Calculated from the day of the year and geographical location. Name: <b>corr.sun</b>. Description: Solar elevation angle (sunheight) adjusted for the angle of sunrise, which changes during the survey. corr.sun = sunheight-sunrise. Name: <b>bottom_temp</b>. Description: Temperature (degrees Celsius) measured near the seafloor in the CTD cast taken closest to the trawl station. Name: <b>dist.CTD.nmi</b>. Description: distance from the trawl station to the nearest CTD cast, in nautical miles. Name: <b>cod.tot</b>. Description: Swept area density of cod &lt; 30 cm in the trawl haul (individuals/km<sup>2</sup>). Name: <b>best.ak</b>. Description: Start date and time of the acoustic segment with which the trawl haul had the highest temporal overlap. Name: <b>capelin.sa</b>. Description: acoustic backscatter (Nautical Area Scattering Coefficient, NASC; m<sup>2</sup>/nmi<sup>2</sup>) of capelin from the acoustic segment with highest temporal overlap with the trawl station (best.ak). Integrated in the water column. Name: <b>SA.per.ind</b>. Description: Acoustic backscatter per capelin individual estimated from the capelin length distribution in the closest pelagic trawl haul and the relationship between length and acoustic target strength. Name: <b>cap.ind</b>. Description: Capelin density in individuals/km<sup>2</sup> (capelin.sa divided by sa.per.ind). Integrated in the water column. Name: <b>capdepth.med</b>. Description: Weighted median depth of the capelin acoustic backscatter, calculated from backscatter resolved in 10 m depth channels. Name: <b>corr.capdepth</b>. Description: capelin weighted median depth adjusted so that it is not deeper than the bottom depth at the start of trawling. This can happen since the acoustic transect covers 1.9 km, over which the bottom depth may change. Name: <b>pred.capdepth</b>. Description: Predicted weighted median depth of capelin acoustic backscatter at median solar elevation angle and survey day. See main article for details. Name: <b>se.pred.capdepth</b>. Description: Standard error of the capelin depth prediction. Name: <b>fish.id</b>. Description: Unique identifier for each cod individual. Name: <b>ind_weight_kg</b>: Description: Weight of the cod individual in kilograms. Name: <b>ind_length_m</b>: Description: Length of the cod individual in meters. Name: <b>age</b>: Description: age of the cod individual in years estimated from otolith reading. Name: <b>sex</b>: Description: Sex of the cod individual. 1: female, 2: male, NA: not determined. Name: <b>tot.weight.prey</b>. Description: Total weight (wet mass, kilograms) of all prey items found in the cod stomach. Name: <b>n.prey</b>. Description: Number of prey items found in the cod stomach. Name: <b>totcap</b>. Description: Total weight (wet mass, kilograms) of capelin prey found in stomach. Name: <b>which.cap</b>. Description: Whether the cod individual has fed on capelin only, other prey only, or capelin and other prey. Name: <b>wdigcap</b>. Description: Mean digestion stage of capelin prey weighted by prey mass (in cases where the cod had capelin of different digestion stages in its stomach). The digestion stage ranges from 1 to 5, where 1 is newly eaten prey and 5 is prey that is completely digested and usually cannot be identified. Heavily digested capelin prey may sometimes be identified based on its particular smell. Name: <b>capelin.consumption.rate</b>. Description: Estimated consumption rate of capelin based on a stomach evacuation model fitted to the mass capelin in the stomach (totcap), bottom temperature (bottom_temp), cod mass (ind_weight_kg*1000) and an evacuation constant specific to capelin prey (see main article). Name: <b>capelin.consump.scaled</b>. Description: Capelin consumption rate per g of cod (capelin.consumption.rate divided by ind_weight_kg*1000). Response variable in the analysis. Name: <b>st.capind</b>. Description: Capelin density (cap.ind) standardized to z-scores. Used as a covariate in the analysis. Name: <b>st.pred.capdepth</b>. Description: Predicted capelin weighted median depth (pred.capdepth) standardized to z-scores. Used as a covariate in the analysis. Name: <b>st.codden</b>. Description: Cod density (cod.tot) standardized to z-scores. Used as a covariate in the analysis. Name: <b>st.codlength</b>. Description: Cod length (ind_length_m) standardized to z-scores. Used as a covariate in the analysis. Name: <b>st.depth</b>. Description: Bottom depth (bottomdepthstart) standardized to z-scores. Used as a covariate in the analysis.