DOI: 10.5061/dryad.mpg4f4rc0
title: More but smaller: Marine heatwaves exacerbate size truncation in overfished fish communities in the Skagerrak

description: As we are progressing through the Anthropocene, it is imperative to
 understand how ecosystems degraded by historical impacts respond to future
 pressures. Particular attention should be directed at changes in the
 distribution and function of foundation species due to their
 disproportionately important ecological roles. Forest-forming kelps are in
 decline and are being replaced by turf-forming algae in coastal areas
 globally, as a consequence of increasingly severe marine heatwaves (MHW).
 This replacement represents a miniaturization of the three-dimensional
 habitat and a change in habitat functions for many associated species.
 Ecologically, economically, and culturally important coastal fish species
 depend on kelp forests in different life stages from egg to adult and may
 be affected in various ways. Many of these species already suffer from
 centuries of overexploitation, severely reducing abundances, truncating
 sizes, and changing age distributions. Here, we studied an overfished
 coastal ecosystem and assessed the impact of the transition from kelp to
 turf on the structure and function of associated fish communities.
 Specifically, we analyzed differences in abundance, body size, and
 composition of benthic fish at sites with variable cover of kelp and turf.
 We found higher abundances of small fish with increasing cover of turf
 relative to kelp. We also found smaller fish within the common families of
 gobids and gadids. Thus, MHW exacerbate size truncation and trophic
 downgrading in coastal fish communities. However, no differences in
 abundances of fish in the water column were detected during the night, and
 the average size of fish at turf-dominated sites was even larger. We
 interpreted this as roving predatory fish during night-time being less
 affected by the shift in habitat and smaller-bodied prey avoiding the
 water column in open turf habitats.
description: # More but smaller: Marine heatwaves exacerbate size truncation in
 overfished fish communities in the Skagerrak Dataset DOI:
 [10.5061/dryad.mpg4f4rc0](https://doi.org/10.5061/dryad.mpg4f4rc0) ##
 Description of the data and file structure This data is the edited version
 of the raw data that was obtained from a) acoustic readings and b)
 analysis of stereo-video rigs using Event Measure.  The general method of
 collecting the data is described in the main paper ### Files and variables
 ### **File: All_WBAT_fish_Dryad_submission-1.csv** **Description:**  Raw
 data from acoustic readings that was used to generate the dataset for
 analysis used in the paper **Variables** * Date:  Written in the following
 format: yyyy.mm.dd * Site: The general location where the acoustic
 readings were done. * Depth:  Exact depth (in m)  below the surface of the
 unit when the readings were done.  * Kelp_percentage:  Percentage cover of
 kelp measured an underwater remotely operated vehicle (ROV)  along a fixed
 transect. * Turf_percentage: Percentage cover of turf measured an
 underwater remotely operated vehicle (ROV)  along a fixed transect. *
 TKratio:  The ratio of turf relative to kelp based on ROV observations.  *
 TSC: Average Target strength. This is a measure (decibels, dB) of the area
 of an acoustic target * Weights: Statistic weighting estimators assigned
 to observations in order to compensate for weighted means. **File:
 Bolærne_3D_measurements_Max_N_Dryad_submission-translated.csv**
 **Description:**  This is edited .csv version of the system generated EM
 observation file (EMObs). **Variables** * Filename:  The unique identifier
 of the video file * Frame: The unique number of the video frame where the
 observation was made * Time (mins): Timestamp on when the observation was
 made * Period: Usually 1 hour * Period time (mins): The time stamp in the
 period when the observation was made * Precision (mm): metadata generated
 by Event Measure (EM) on the precision of the length measurement * Range
 (mm): metadata generated by EM on the range of the point measurements *
 Direction (deg): metadata generated by EM on the direction of the length
 measurement * Horz. Dir. (deg): metadata generated by EM on the direction 
 of the measurement based on the horizontal line * Vert. Dir. (deg):
 metadata generated by EM on the direction of the measurement based on the
 vertical line * Mid X (mm):  location of the point/measurement in the X
 axis * Mid Y (mm): location of the point/measurement in the y axis * Mid Z
 (mm): location of the point/measurement in the z axis * OpCode: unique
 identifier of the station * TapeReader: person who did the analysis *
 Depth: depth to which the BRUV unit was deployed, defined in meters *
 Comment: general comment, usually about the habitat * Date:  coded as
 day.month.year * Time start:  When the recording was started * Time stop:
 when the recording was stopped * Station number: unique identifier of the
 station * Area: general area or location * Lat:  latittude * Long:
 longitude * Duration (min):  actual duration of film recording * Family:
 fish family observed  * Genus: genus of the fish identified * Species:
 species of the fish identified * Code:  NANSIS code of the fish identified
 * Number:  number of fish observed, this is not necessarily the maximum *
 Length (mm):  the measured length of a fish from snout to edge of tail *
 MaxN: the maximum number of a fish species observed in one frame * Stage:
 not used, system generated variable by EM * Activity: not used, system
 generated variable from EM ## Code/software Event Measure and R was used
 in generating this data set. R and Notebook/excel, any program that can
 read en .csv file can be used in reading and exploring this data set. ##
 Access information Other publicly accessible locations of the data: * IMR
 Flødevigen Data was derived from the following sources: * This is original
 data