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Both active and passive human interactions with reef fish communities are increasingly recognized to cause fish behavioural changes. However, few studies have considered how these behavioural adaptations impact standard reef survey techniques, particularly across natural gradients of interest to ecologists and reef managers. Here we measure fish abundance, biomass and minimum approach distance using stereo-video surveys to compare the effects of bubble-producing open-circuit scuba vs near-silent closed-circuit rebreathers. Surveys extended across a shallow to upper-mesophotic gradient on the fringing reefs of Utila, Honduras, to explore how the effects of diver gear choice vary with depth. For most fish families we recorded similar abundances and biomass with the two diving techniques, suggesting that open-circuit transects are generally appropriate for surveying western Atlantic reefs similar to Utila with regular tourist diving but no spearfishing. Despite no overall significant difference in fish abundance or biomass, we identified several fish families (Labridae, Pomacentridae, Scaridae) that allowed closed-circuit rebreather divers to approach more closely than open-circuit divers. In addition, smaller fish generally allowed divers to approach more closely than larger fish, and in most cases divers could approach fish more closely on mesophotic than shallow reefs. Despite these significant differences in approach distances, their magnitude suggest they are unlikely to affect reef fish detectability during normal fish surveys for most families. Our findings highlight the importance of considering variation in fish behavioural adaptations along natural gradients such as depth, which otherwise has the potential to cause biases when surveying by traditional monitoring programmes.
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Wariness of reef fish to passive diver
presence with varying dive gear type
across a coral reef depth gradient
dominic a. andradi-brown
, erika gress
, jack h. laverick
, margaux a. a. monfared
alex david rogers
and dan a. exton
Department of Zoology, University of Oxford, The Tinbergen Building, South Parks Road, Oxford OX1 3PS, UK,
Wallacea, Wallace House, Old Bolingbroke, Spilsby PE23 4EX, UK,
School of Biological Sciences, University of Portsmouth,
University House, Winston Churchill Avenue, Portsmouth PO1 2UP, UK
These authors have contributed equally and are joint last authors.
Both active and passive human interactions with reef fish communities are increasingly recognized to cause fish behavioural
changes. However, few studies have considered how these behavioural adaptations impact standard reef survey techniques,
particularly across natural gradients of interest to ecologists and reef managers. Here we measure fish abundance, biomass
and minimum approach distance using stereo-video surveys to compare the effects of bubble-producing open-circuit scuba vs
near-silent closed-circuit rebreathers. Surveys extended across a shallow to upper-mesophotic gradient on the fringing reefs of
Utila, Honduras, to explore how the effects of diver gear choice vary with depth. For most fish families we recorded similar
abundances and biomass with the two diving techniques, suggesting that open-circuit transects are generally appropriate for
surveying western Atlantic reefs similar to Utila with regular tourist diving but no spearfishing. Despite no overall significant
difference in fish abundance or biomass, we identified several fish families (Labridae, Pomacentridae, Scaridae) that allowed
closed-circuit rebreather divers to approach more closely than open-circuit divers. In addition, smaller fish generally allowed
divers to approach more closely than larger fish, and in most cases divers could approach fish more closely on mesophotic than
shallow reefs. Despite these significant differences in approach distances, their magnitude suggest they are unlikely to affect
reef fish detectability during normal fish surveys for most families. Our findings highlight the importance of considering vari-
ation in fish behavioural adaptations along natural gradients such as depth, which otherwise has the potential to cause biases
when surveying by traditional monitoring programmes.
Keywords: Fish wariness, mesophotic coral ecosystem, closed-circuit rebreather, flight initiation distance, passive diver presence, DOV,
stereo-video system, minimum approach distance, Honduras
Submitted 17 June 2016; accepted 6 June 2017
Many studies have assessed the effectiveness of marine pro-
tected areas, including different management forms such as
no-take zones and partial protection, generally finding that
marine protected areas are effective in maintaining fish
density and biomass (Sciberras et al., 2013). However, assess-
ments rarely consider whether varying fish behaviours across
the study area may bias the results of their survey techniques
(Kulbicki, 1998, Feary et al., 2011). Fish behaviour is known to
be impacted by previous exposure to humans (Januchowski-
Hartley et al., 2015), yet locations with direct human interac-
tions with the marine environment tend to be those reef
managers are most interested in assessing. Even on a local
scale, the exposure of fish communities to these effects can
be highly variable along natural gradients such as depth.
With much recent interest in the threats faced by mesophotic
coral ecosystems (MCEs; reefs 30 150 m depth) (Andradi-
Brown et al., 2016a), and whether they act as refuges from
fishing (Bejarano et al., 2014; Lindfield et al., 2016), gaining
a better understanding of fish behavioural survey biases
across depth gradients is crucial.
While bias in some form is an unavoidable symptom of all
survey methods, stakeholders rely heavily on data pertaining
to fish populations and their responses to management inter-
ventions to inform decision-making. On tropical coral reefs
this tends to involve baseline fish community data collected
using underwater visual census (UVC) by surveyors in the
water (English et al., 1997; Sale, 1997; Mapstone & Ayling,
1998). In many cases, to conduct UVC open-circuit (OC)
scuba divers swim along a fixed-length transect recording
individuals of all (or target) fish species, additionally estimat-
ing lengths in some cases (English et al., 1997). As a result of
concerns about repeatability between surveyors because of
observer bias (Thompson & Mapstone, 1997), video surveys
have begun to replace in-water observations for many
surveys (Mallet & Pelletier, 2014). While the use of video
removes many errors associated with in-water data collection,
the differing response behaviours of reef fish to diver presence
Corresponding author:
D.A. Andradi-Brown
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raises new concerns (Chapman et al., 1974; Cole, 1994;
Watson & Harvey, 2007). The operation and bubble release
of traditional OC scuba generates high levels of noise at the
low frequencies fish are most sensitive to (Radford et al.,
2005). This has led to the suggestion fish may be able to
audibly detect diver presence before divers can visually iden-
tify fish (Radford et al., 2005). This potentially could allow
fish to avoid areas of reef with divers present and so remain
undetected, or aggregate around divers in the water from a
larger reef area enhancing fish abundance and biomass
Despite these known effects, few UVC studies acknowledge
the bias that OC scuba is likely to cause to their results
(Dickens et al., 2011), instead most biases are assumed to be
consistent across survey sites and thus mitigated. However,
in areas with regular spearfishing, flight initiation distance
(FID; the minimum distance a diver can approach a fish
before it flees) of many fish families have repeatedly been
found to be higher than in protected areas (Gotanda et al.,
2009; Januchowski-Hartley et al., 2011,2014). Therefore,
despite standardized methodologies, varying fishing pressure
may bias survey results and artificially inflate the apparent
effectiveness of marine protected areas (Lindfield et al.,
2014a). For example, Lindfield et al.(2014a) used diver-
operated stereo-video systems to assess how close they could
approach fish when surveying using OC vs CCR. They
observed that fish species targeted by spearfishers avoid OC
divers, and this was significant enough to reduce abundance
and biomass estimates compared with surveys conducted in
the same location using CCR. Even within protected areas,
other more subtle effects may significantly bias results, for
example fish habituation to the presence of divers in areas
with intensive dive tourism (Titus et al., 2015), which can
be enhanced when divers feed fish populations (Cole, 1994),
or the presence of fish ontogenetic migrations, with more
mature and thus larger individuals found at greater depths
(Grol et al., 2014). Many fish families exhibit greater FID in
larger individuals than smaller individuals (Gotanda et al.,
2009; Januchowski-Hartley et al., 2011), suggesting for
species with well-defined ontogenetic migrations, divers
might be able to approach individuals more closely on
shallow reefs than deeper reefs.
The effects of recreational dive tourism are highly depth
biased, being typically limited to a maximum depth of 30 m,
and in reality generally much shallower because of training
limitations and no-decompression limits. This subsequently
skews the opportunity for fish habituation to OC divers
towards shallower reefs. Even when surveying across depth
gradients, the increased distance for bubbles to travel to the
surface when completing deeper transects is likely to produce
greater total noise (Radford et al., 2005), and thus impacts
on fish behaviour, than shallower transects, leading to unin-
tended bias in the data. Recent advances have made technical
diving more accessible, including the emergence of commer-
cially available Closed-Circuit Rebreathers (CCR). CCR
systems recycle gas rather than releasing bubbles into the
water column, see Sieber & Pyle (2010) for a detailed system
overview, and are therefore significantly quieter than their
OC counterparts. When it comes to their impact on fish behav-
iour, fish can detect the sounds of OC divers .200 m away
over a range of typical background noise levels, while CCR
divers can only be detected between 0.3 15.9 m away depend-
ing on background underwater noise (Radford et al., 2005).
It therefore seems likely that some patterns detected in fish
surveys using OC divers across depth gradients may be influ-
enced by fish behavioural biases that change with depth. Yet
no studies have directly investigated how detection bias in
OC scuba changes across the depth gradient. To investigate
this question we conducted fish community assessments
across a shallow to upper-mesophotic reef gradient at three
sites in the Bay Islands Marine Park, Utila, Honduras using
OC scuba and CCR. By conducting surveys using CCR, fish
disturbance effects caused by bubbles and sounds of OC
scuba regulators were absent, reducing these biases on fish
community assessment. We investigated whether depth inter-
acted with the differences observed between the two methods.
As the shallow reefs of Utila are heavily dived by recreational
divers, mesophotic fish populations are likely to be less habi-
tuated to diver presence than shallow reef fish, suggesting that
individuals could be more wary of OC divers on MCEs than
shallow reefs.
Study sites
Surveys were conducted on the south shore of Utila, Bay
Islands, Honduras (Figure 1). Utila is located 29 km off
the Caribbean coast of Honduras, forming the southern
extent of the Mesoamerican Barrier Reef (Harborne et al.,
2001). While Utila is within the Bay Islands Marine Park,
fishing is allowed on the reefs, with the majority of fishing,
including at our study sites, carried out by handlines targeting
Lutjanidae and Serranidae (Gobert et al.,2005; Box & Canty,
2011). Historically fishers carried spears to opportunistically
shoot large fish they encountered, however, spearfishing has
been banned from the island’s fringing reefs since 2004
(Kramer et al., 2015). Recently tourism has replaced fishing
as the dominant source of income (Cronk & Steadman,
2002; Doiron & Weissenberger, 2014), primarily consisting
of recreational diving (Doiron & Weissenberger, 2014), with
.10 dive centres operational on Utila and tens of thousands
of recreational dives completed annually. Therefore, while
our study sites are fished at a relatively low level, they are
very heavily dived by recreational divers.
Fig. 1. Map of Utila with the three survey sites marked. (1) Little Bight, (2)
Coral View and (3) Rocky Point. Inset map indicates location of Utila
relative to the western Atlantic region.
2 dominic a. andradi-brown et al.
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Shallow reefs on the south shore of Utila are a spur and
groove system, which transition into a mesophotic patch
reef system at 30 40 m depth. Therefore shallow reefs
exist as a continuous reef system with 92% hard substratum
cover, while MCEs are broken by areas of sand, so only have
20% hard substratum cover (Andradi-Brown et al., 2016b).
Reef fish abundance and biomass patterns across this shallow
to upper-MCE depth gradient on the south shore of Utila have
previously been studied, finding that for the majority of
species both abundance and biomass decline with increased
depth (Andradi-Brown et al., 2016b).
Video surveys of fish communities
Fish community surveys were conducted along 50 m long by
5 m wide transects using a diver operated stereo-video system
(DOV), made up of two Canon HFS21 video cameras (see
Andradi-Brown et al., 2016b). Cameras were separated by
0.75 m with convergence angles of 888888. By recording with
two cameras simultaneously, DOV allows post-dive analysis
of fish abundance and accurate measurements of body
length and distance from camera, all within automatically
defined transect boundaries (Harvey et al., 2001,2004). Four
replicate transects separated by 10 m were conducted follow-
ing the reef contour at four depths (5, 15, 25, 40 m) at three
sites, giving 48 unique transect locations across all sites and
depths (Figure 1). Each transect was surveyed twice, with
each survey taking place on different days, once using OC
scuba equipment and once using CCR, giving 96 transect
surveys conducted in total. The order of whether to survey a
transect by CCR or OC first was randomized for each individ-
ual transect. In no cases were all four transects at the same
depth at a site surveyed by the same dive gear type within
the same day. All survey sites had fixed/marked start locations
for each transect allowing the same area of reef to be surveyed
by both methods. All transects were conducted during
daytime between the hours of 8 am and 4 pm during July
September 2015. Transects were surveyed by a DOV operator
using OC or CCR followed by a second diver laying a transect
tape to measure distance. To minimize disturbance to the fish
community prior to recording, a 60 m transect tape was used,
with the cameras set recording and synchronized with a hand
torch before swimming 10 m along the reef with the cameras
filming directly downwards below the diver. After the DOV
operator had swum 10 m, the transect diver signalled for
them to lift the cameras and begin the transect proper, signal-
ling again once the full 50 m had been swum. Cameras were
held to film looking forward along the reef following the
depth contour. Each transect took 3 min to film. CCR
surveys were conducted using a mixture of Hollis Prism 2
(Hollis, San Leandro, CA, USA), rEvo X micro (rEvo
rebreathers, Bruges, Belgium), or Sentinel (Vobster Marine
Systems, Somerset, UK) rebreathers.
Video analysis
Transect videos were blinded to survey method and analysed
with EventMeasure software v3.51 (SeaGIS, Melbourne,
Australia). Transect boundaries were defined as 2.5 m either
side of the transect giving a 5 ×50 m survey area for each
transect. All fish were identified to species level or the
lowest taxonomic level possible if not identifiable to species.
If visible on both cameras, fish length and distance from
cameras was recorded using the built in measurement tools
in EventMeasure. Measurements were taken when each indi-
vidual fish was at its closest to the DOV system, thus recording
the minimum approach distance (MAD; the minimum dis-
tance the DOV operator could approach the fish before it
moved away). Where MADs were below the minimum dis-
tance needed to appear simultaneously on both cameras, we
recorded the fish at the closest point while visible on both
cameras. The exact minimum distance for a fish to appear
on both cameras was variable based on how far from the
centre of the transect the fish was located. For a fish central
to the transect, the minimum MAD that could be recorded
is 50 cm. All visible fish in front of the cameras, regardless
of distance away, and within the 5 m transect width were
recorded at the point they were closest to the DOV. Fish
lengths were converted to biomass using length-weight para-
meters for each species from Fishbase (Froese & Pauly,
2016). Where fish appeared on the transect but were never
visible on both cameras simultaneously, fish lengths and
MAD were unable to be recorded and thus these were only
included in abundance and biomass analysis. To estimate
biomass for these fish, they were first allocated the mean
length recorded for that species from other individuals
recorded on the transect and this estimate then converted to
biomass. Raw data is available from figshare (http://dx.doi.
org/10.6084/m9.figshare.5072329). Data analyses were con-
ducted at the family level. However, because of the historical
large bodied grouper fishery around Utila we split the family
Serranidae into the two sub-families: Epinephelinae and
Serraninae. In addition, because of the differing ecological
roles on Caribbean reefs, we considered the Scaridae sub-
family separately from all other members of the Labridae
family during analysis.
Abundance and biomass analysis
As a result of the non-normal nature of fish abundance data
we used permutational multivariate analyses of variance
(PERMANOVA) because PERMANOVA has fewer assump-
tions about the distribution of the data (Anderson et al.,
2008). A PERMANOVA was run for each fish family indi-
vidually based on a Bray Curtis dissimilarity matrix con-
structed with the square rooted abundance data or biomass
data, testing for differences between sites, depth and the site:
depth interaction. Bray– Curtis was used so common absences
of fish families from transects do not influence the results
(Clarke et al., 2006), because we had many transects where a
fish family was not recorded by either CCR or OC.
However, Bray Curtis dissimilarities are undefined when
transects contain no individuals at all. To address this, we
included an additional dummy family given an abundance
or biomass of 1 for all transects alongside the family of interest
in all PERMANOVAs (Clarke et al., 2006). We restricted ana-
lysis to fish families that appeared on .50 transect surveys
(out of 96 total) across all sites and depths. All
PERMANOVAs were conducted in R (R Core Team, 2013)
using the ‘adonis’ function in the package vegan (Oksanen
et al., 2013) and run for 99,999 permutations.
Minimum Approach Distance (MAD) analysis
To investigate MAD for each fish family we used analysis of
covariance (ANCOVA). Differences in MAD between
dive gear and depth effects on reef fish 3
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methods are most likely to affect the detection of fish furthest
away from the divers. To identify whether differences in MAD
potentially could alter fish detection ability over the distances
visually surveyed by DOV, we calculated the 90th quantile of
MAD for each fish family by transect for families that were
detected on .50 transects. To meet normality and homogen-
eity assumptions we natural log-transformed raw MAD data.
We tested the effects of site (categorical), depth (continuous)
and method (categorical) on MAD. For significant explana-
tory variables we used the model to generate predictions
based on altering the significant explanatory variable, and
then back-transformed log(MAD) predictions to see the
effect on MAD.
As individual fish body size has been show to alter fish
behavioural responses to divers, we also analysed our data
using an ANCOVA without grouping fish family data by tran-
sect. This allowed four explanatory variables: site and survey
method (categorical), and depth and fish length (continuous),
and was done for all fish families with 30 individual MAD
and fish length observations for both CCR and OC (giving a
minimum total number of observations of 60). We also
tested for interactions between survey method, depth and
fish length on MAD. ANCOVA models were fitted in base
R using the stats package (R Core Team, 2013). The function
‘step’ was used to simplify models starting with the full model
with all interactions, and iteratively removing one variable or
interaction at a time from the model starting with the most
complex. If removing a variable or interaction resulted in a
lower model Akaike information criterion (AIC) the variable
or interaction was dropped, if not it was replaced and
another variable or interaction tested. Where significant
two-way interactions were identified we examined them
while controlling for the other explanatory variables by fol-
lowing a partial correlation approach (Brown & Hendrix,
2014). This involved plotting relationships between the
centred residuals from linear models of the variables of inter-
est with the other explanatory variables. During model check-
ing two Lachnolaimus maximus records were removed as they
had an undue influence on the fitted model. Lachnolaimus
maximus is a large-bodied Labridae species and these two
records, the only times we recorded this species, were both
of individual fish substantially larger than any other
Labridae in our dataset. With these two individuals removed
the remaining 379 Labridae MAD and length measurements
met the model assumptions.
Median fish abundance was greater for CCR transects than
OC at all surveyed depths (Figure 2), however there was no
significant effect of survey method on total fish abundance
per transect (PERMANOVA, Pseudo-F¼2.17, P¼0.09) or
total fish biomass per transect (PERMANOVA, Pseudo-F¼
0.01, P¼0.96). However, we found abundance differences
between CCR and OC when considering specific fish families.
Method effects were recorded based on abundance for
Acanthuridae and Tetraodontidae, and based on biomass for
Lutjanidae and Tetraodontidae. In addition we found signifi-
cant method:depth interactions based on abundance and
biomass for Tetraodontidae across the shallow to mesophotic
depth gradient (Table 1). Acanthuridae median abundance
per transect was similar between methods (Figure 3A), but
shallow (5 m) OC transects detected several large schools of
Acanthurus coeruleus increasing the mean abundance esti-
mates from 1.83 +0.58 per 250 m
for CCR to 6.67 +5.05
per 250 m
for OC (mean +SE). Tetraodontidae displayed
similar abundance recorded by both methods in the shallows,
but greater abundance recorded by CCR than OC at 25
(14.92 +3.55 vs 5.83 +5.21 per 250 m
) and 40 m
(25.67 +13.09 vs 4.17 +3.92 per 250 m
)(Figure 3B).
Tetraodontidae biomass also showed the same pattern
(Figure 3D). Greater Lutjanidae biomass was recorded by
CCR in the shallows than OC, with 637 +279 vs 463 +
297 g per 250 m
at 5 m while there was less difference
between CCR and OC (139 +38 vs 309 +110 g per
250 m
)at40m(Figure 3C). The majority of fish families
tested did not show any effect of survey method on abundance
or biomass, though many exhibited effects of depth and survey
site on abundance (Table 1). No difference in abundance or
biomass between the two methods was detected for
Haemulidae, Labridae, Lutjanidae, Pomacentridae or
Scaridae (Table 1).
Minimum Approach Distance (MAD)
To evaluate whether fish are likely to be missed from transects
we examined the 90th quantile of MAD for each fish family by
transect. Only Scaridae showed any effect of method, with indi-
viduals fleeing more readily from OC than CCR divers
(Table 2). However, Labridae, Lutjantidae, Pomacentridae
and Scaridae all showed effects of depth on the 90th quantile
of MAD, with fish allowing divers to more closely approach
at 40 m compared with the shallows (Table 2). Despite these
broad patterns when aggregating data across whole transects,
when considering individual fish, differences in MAD in rela-
tion to survey method, depth and individual fish length were
observed. Table 3 shows analysis results for individual fish,
along with model predictions of MAD. MAD was greater for
Fig. 2. Comparison of the two survey methods across the depth gradient for
total fish abundance per transect for all sites. The solid black line represents
the median, with the box indicating the upper and lower quartiles, and
whiskers representing the maximum or minimum observed value that is
within 1.5 times the interquartile range of the upper or lower quartile,
respectively. Open circles represent data points that fall outside the mean +
1.5 times the interquartile range.
4 dominic a. andradi-brown et al.
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OC for Labridae, Pomacentridae and Scaridae, meaning indi-
vidual fish could be more closely approached 23%, 6% and
14% respectively by divers when using CCR (Table 3). The sub-
family Serraninae showed the opposite pattern, with divers able
to approach 17% closer on OC than CCR (Table 3).
Interactions between method and depth were also found for
Labridae, with divers on average able to approach fish more
closely on CCR (229.3 +48.2 cm) than OC (264.2 +
34.9 cm) at 5 m. However, this difference decreased as depth
increased, with little difference between CCR (194.9 +
44.6 cm) and OC (168.0 +34.7 cm) at 40 m (Figure 4D).
Many fish families did not show a difference in MAD
between the two survey methods, including Acanthuridae,
Hamulidae, Lutjanidae and Tetraodontidae (Table 3).
Many families were observed to have higher MADs for
larger individuals, including Acanthuridae, Labridae,
Pomacentridae and Scaridae, meaning larger fish allowed
divers to approach them less closely than smaller fish
(Table 3). In some cases these differences were large, for
example, we predict on shallow reefs using OC a juvenile
Acanthuridae 9.9 cm long would have a MAD of 205.9 cm,
while a mature individual 23.2 cm long would have a MAD
of 290.6 cm giving .0.8 m difference. Haemulidae also
showed a significant depth and length interaction, with
greater effects of fish length on MAD at mesophotic depths,
and less effect in the shallows (Figure 4A), suggesting that
larger individuals are more tolerant of allowing divers closer
on shallow reefs than they are at mesophotic depths.
We also detected interactions between fish length and
survey method for Labridae and Pomacentridae (Table 3),
with smaller Labridae appearing more wary of CCR divers
than OC divers which reversed for larger individuals, which
were more wary of OC divers (Figure 4C). However, the
opposite patterns was observed for Pomacentridae
(Figure 4B). For Haemulidae we detected a Length:Depth
interaction, with larger fish being more wary on deeper reefs
than smaller fish, yet less difference based on body size in
the shallows (Figure 4A).
Of the seven fish families encountered on more than 50 tran-
sect surveys, only three showed variations in overall abun-
dance or biomass between OC and CCR, and we found no
difference in total fish abundance or biomass between
the two methods. While we did detect differences in fish
Table 1. PERMANOVA results based on Bray Curtis dissimilarity
matrix for difference in fish abundance and biomass recorded for each
fish family by OC and CCR.
Abundance Biomass
DF Pseudo-Fp(perm) Pseudo-Fp( perm)
Site 2 4.69 <0.001 4.47 <0.001
Method 1 3.79 0.045 1.93 0.151
Depth 1 32.22 <0.001 23.99 <0.001
Method:Depth 1 0.70 0.434 1.09 0.301
Residuals 90
Total 95
Site 2 0.33 0.190 0.34 0.223
Method 1 3.06 0.079 3.23 0.064
Depth 1 0.78 0.385 0.21 0.745
Method:Depth 1 0.91 0.345 0.86 0.367
Residuals 90
Total 95
Site 2 12.10 0.001 5.21 0.006
Method 1 0.15 0.834 0.15 0.917
Depth 1 13.99 <0.001 9.06 <0.001
Method:Depth 1 0.09 0.897 0.23 0.845
Residuals 90
Total 95
Site 2 0.87 0.244 1.46 0.154
Method 1 2.84 0.091 4.10 0.031
Depth 1 1.27 0.259 0.56 0.508
Method:Depth 1 0.08 0.847 0.39 0.615
Residuals 90
Total 95
Site 2 14.34 <0.001 9.87 <0.001
Method 1 0.31 0.691 0.63 0.544
Depth 1 80.69 <0.001 49.73 <0.001
Method:Depth 1 1.96 0.147 2.49 0.077
Residuals 90
Total 95
Site 2 8.62 <0.001 6.85 <0.001
Method 1 0.16 0.773 0.33 0.724
Depth 1 26.32 <0.001 16.92 <0.001
Method:Depth 1 0.38 0.586 0.24 0.820
Residuals 90
Total 95
Site 2 11.85 <0.001 10.97 <0.001
Method 1 11.84 <0.001 8.66 0.001
Depth 1 7.20 0.005 5.66 0.011
Method:Depth 1 14.47 <0.001 13.76 <0.001
Residuals 90
Total 95
Only fish families recorded on .50 out of the 96 transect surveys are
Fig. 3. Fish family abundance or biomass recorded by CCR (light grey) and
OC (dark grey) across the depth gradient. Fish families are (A)
Acanthuridae abundance, (B) Tetraodontidae abundance, (C) Lutjanidae
biomass and (D) Tetraodontidae biomass. The solid black line represents the
median, with the box indicating the upper and lower quartiles and whiskers
representing the maximum or minimum observed value that is within 1.5
times the interquartile range of the upper or lower quartile respectively.
dive gear and depth effects on reef fish 5
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behaviour between OC scuba and CCR techniques, particu-
larly Minimum Approach Distance (MAD) for Labridae,
Pomacentridae and Scaridae and Serraninae, only Scaridae
showed differences between methods for 90th quantile of
MAD (representing fish remaining furthest away from the
cameras). Despite this, none of these differences in MAD
appear to be of sufficient magnitude to affect detectability
during typical reef monitoring programmes. This suggests
generally OC scuba surveys are appropriate for reef fish com-
munity monitoring on Utila.
Detectability variation between OC and CCR
Of the differences in abundance and biomass we identified
between techniques: Tetraodontidae abundance and biomass
was similar on shallow reefs between techniques, but greater
abundance and biomass was recorded by CCR than OC on
MCEs. Lutjanidae biomass was higher when surveyed with
CCR than OC. In contrast Acanthuridae abundance was
similar between methods on MCEs, yet OC recorded more
Acanthuridae individuals in the shallows. To our knowledge
only two studies have previously compared fish abundance
and biomass surveys conducted by OC and CCR, both of
which were located in the Indo-Pacific region and focused
on comparing fish surveys at the same depth between areas
with differing levels of spearfishing (Lindfield et al., 2014a;
Gray et al., 2016). One conducted in Micronesia demonstrated
a clear negative effect of OC on recorded fish biomass in areas
with spearfishing (Lindfield et al., 2014a). The other, con-
ducted in Hawaii found no overall effect across a gradient in
fishing pressure, but that at the most heavily spearfished
site, fish biomass of key species was lower when surveyed by
OC (Gray et al., 2016). Therefore we believe this study is the
first identifying that differences in fish community surveys
between OC and CCR can be depth specific.
We found few differences in abundance or biomass
between OC and CCR for most fish families. This lack of
major differences between survey methods for many fish fam-
ilies’ abundance and biomass was surprising, as it is in con-
trast to previous studies which demonstrate effects of diver
presence in the water, e.g. Watson & Harvey (2007).
However, Watson & Harvey (2007) used fish point counts
from a static camera system with an OC diver either present
or absent. Habituation to OC divers has been recorded in
reef fish on Utilan fringing reefs, though habituated fish still
exhibited diminished behaviours compared with surveys
without divers present (Titus et al., 2015). Previous studies
have also found few differences when comparing fish commu-
nities surveyed by OC and semi-closed rebreathers (Cole et al.,
2007), though unlike CCR semi-closed rebreathers produce
bubbles. Our lack of differences in observed abundance or
biomass between survey techniques for many fish families
fits with results of Lindfield et al.(2014a), who found that
their non-spearfished family control, Chaetodontidae, did
not show a significant difference in biomass between survey
techniques, and also similar results between methods within
protected areas. The lack of difference in abundance or
biomass based on survey method is further supported by
our analysis of the 90th quantile of MAD for seven fish fam-
ilies. We found only one family, Scaridae, which had a lower
quantile of MAD for CCR than OC. This suggests that
regardless of technique, fish with the greatest MAD are still
similar between methods, and within the detection range of
both techniques.
While our abundance and biomass results and 90th quan-
tile of MAD suggest that choice of dive technique has limited
effect on broad fish community results, our MAD results show
that some fish families did in fact exhibit differences in their
responses to the two dive techniques. MADs were lower for
CCR surveyed fish than OC for several families (Labridae,
Pomacentridae, Scaridae). Only the sub-family Serraninae
had smaller MAD for OC transects than CCR transects.
This leads us to conclude that differences between OC and
CCR do cause detectable effects on fish surveys, most likely
through driving active avoidance or attraction behavioural
responses in some key fish families. However, despite these
differences being detectable, they are unlikely to undermine
the data collection and the reliability of either dive gear
Table 2. ANCOVA model results for 90th quantile per transect of log
Minimum Approach Distance (MAD) for fish families following simplifi-
cation based on model AIC.
Family/term DF MS FP Effect on MAD (cm)
Site 2 0.45 2.91 0.066
Depth 1 0.10 0.64 0.427
Method 1 0.61 3.94 0.054
Residuals 43 0.15
Site 2 0.00 0.04 0.961
Depth 1 0.19 2.11 0.154
Method 1 0.18 2.00 0.164
Residuals 45 0.09
Site 2 1.05 7.03 0.002 CV: 263.7, LB: 285.6, RP:
Depth 1 2.53 16.91 <0.001 5 m: 388.4, 40 m: 226.5
Method 1 0.26 1.76 0.190
Residuals 64 0.15
Site 2 0.13 1.06 0.354
Depth 1 0.72 5.72 0.021 5 m: 416.8, 40 m: 307.3
Method 1 0.20 1.56 0.217
Residuals 52 0.13
Site 2 0.64 10.29 <0.001 CV: 309.7, LB: 358.2, RP:
Depth 1 2.29 36.67 <0.001 5 m: 417.2, 40 m: 279.3
Method 1 0.01 0.15 0.699
Residuals 75 0.06
Site 2 0.71 6.45 0.003 CV: 312.7, LB: 334.0, RP:
Depth 1 1.12 10.22 0.002 5 m: 404.8, 40 m: 300.6
Method 1 0.48 4.38 0.040 OC: 386.2, CCR: 339.3
Residuals 73 0.11
Site 2 0.22 2.09 0.133
Depth 1 0.01 0.11 0.744
Method 1 0.04 0.35 0.555
Residuals 60 0.11
Only fish families that were recorded on .50 transects. Effect on MAD
column shows the mean 90th quantile MAD value (in cm) across all trans-
ects within a category for variables that were found to be significant, allow-
ing direction and approximate magnitude of effects to be seen. Sites were:
Coral View (CV), Little Bight (LB) and Rocky Point (RP), while methods
were: open circuit (OC) and closed-circuit rebreather (CCR).
6 dominic a. andradi-brown et al.
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when conducting surveys. Other studies, for example, Feary
et al.(2011) have reached analogous conclusions when com-
paring FID in areas with fishing and those without. They
found that while active spearfishing increases mean FID,
this increased FID does not extend beyond the range at
which the fish can be detected (Feary et al., 2011).
Table 3. ANCOVA model results for log Minimum Approach Distance (MAD) for fish families following simplification based on model AIC.
Family (no. individuals)/term DF MS FP Predicted MAD (cm)
Acanthuridae (78)
Site 2 1.20 7.48 0.001 CV: 204.1, LB: 263.4, RP: 327.7
Depth 1 0.03 0.21 0.646
Method 1 0.37 2.31 0.133
Length 1 1.18 7.36 0.008 10% (9.9 cm): 205.9, 90% (23.2 cm): 290.6
Depth:Length 1 0.43 2.66 0.107
Residuals 71 0.16
Haemulidae (118)
Depth 1 1.52 14.84 <0.001 5 m: 269.5, 40 m: 205.9
Length 1 0.00 0.03 0.876
Depth:Length 1 0.43 4.23 0.042
Residuals 114 0.10
Labridae (373)
Site 2 0.42 3.97 0.020 CV: 184.3, LB: 184.6, RP: 212.1
Depth 1 5.58 52.28 <0.001 5 m: 199.3, 40 m: 151.5
Method 1 1.79 16.82 <0.001 OC: 208.9, CCR: 184.3
Length 1 3.51 32.92 <0.001 10% (3.0 cm): 202.2, 90% (13.1 cm): 219.9
Depth:Method 1 0.52 4.85 0.028
Depth:Length 1 0.03 0.31 0.580
Method:Length 1 0.76 7.12 0.008
Depth:Method:Length 1 0.71 6.68 0.010
Residuals 363 0.11
Lutjanidae (113)
Site 2 0.91 9.58 <0.001 CV: 321.0, LB: 278.7, RP: 286.0
Depth 1 1.01 10.65 0.001 5 m: 347.3, 40 m: 263.7
Length 1 0.06 0.67 0.414
Residuals 108 0.09
Pomacentridae (759)
Site 2 0.55 4.71 0.009 CV: 228.3, LB: 227.3, RP: 254.1
Depth 1 2.31 19.81 <0.001 5 m: 240.4, 40 m: 200.6
Method 1 1.56 13.37 <0.001 OC: 249.6, CCR: 228.3
Length 1 1.62 13.93 <0.001 10% (3.0 cm): 248.2, 90% (10.5 cm): 251.0
Depth:Length 1 0.41 3.52 0.061
Method:Length 1 1.33 11.41 0.001
Residuals 751 0.12
Scaridae (293)
Site 2 0.44 3.50 0.032 CV: 222.0, LB: 220.1, RP: 241.4
Depth 1 0.70 5.58 0.019 5 m: 232.7, 40 m: 197.5
Method 1 1.00 7.96 0.005 OC: 254.1, CCR: 222.0
Length 1 3.98 31.84 <0.001 10% (7.0 cm): 223.3, 90% (26.6 cm): 300.3
Residuals 287 0.13
Serraninae (92)
Method 1 1.04 10.59 0.002 OC: 156.8, CCR: 195.7
Length 1 0.09 0.90 0.344
Method:Length 1 0.27 2.70 0.104
Residuals 66 0.10
Tetraodontidae (296)
Depth 1 1.00 11.53 <0.001 5 m: 200.7, 40 m: 170.8
Method 1 0.18 2.07 0.151
Length 1 0.18 2.10 0.148
Method:Length 1 0.29 3.31 0.070
Residuals 291 0.09
Only fish families that 30 individual fish MAD measurements were made for both open circuit and closed circuit are shown (therefore 60 MAD
measurements per family). Predicted MAD shows the prediction from the fitted model for MAD when altering the variable of interest while holding
all other variables constant. All MAD predictions are reported in cm. For site effects, the prediction was based on assuming 15 m depth, CCR
surveys and the mean fish length for the family recorded within our dataset. Depth effect predictions used 5 and 40 m depths and are based on:
Coral View, open-circuit diving and mean family fish length. Method effect predictions are based on: Coral View, 15 m depth and mean family fish
length. Length predictions are based on Coral View, 15 m depth and open-circuit scuba, the presented length predictions represent the 10 and 90% quan-
tiles of recorded fish lengths for the family (these fish body lengths are indicated in brackets below). Sites were Coral View (CV), Little Bight (LB) and
Rocky Point (RP).
dive gear and depth effects on reef fish 7
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Several possible explanations are likely for the fish families
for which we did observe different abundances or biomass
between techniques. However, caution is required in inter-
preting the results, as surveys were limited to three sites and
reef structure changes across the shallow to upper-MCE
depth gradient (Andradi-Brown et al., 2016b), making inter-
preting Depth:Method interactions harder. Higher abun-
dances recorded for Acanthuridae by OC are likely to be
caused by shallow OC transects encountering several large
schools of Atlantic blue tang (Acanthurus coeruleus) that
were not encountered when the transects were conducted by
CCR. While we cannot rule out attraction effects of OC
divers to these schools, we did not observe any other fish fam-
ilies attracted to OC on shallow reefs.
Tetraodontidae abundance patterns are harder to explain,
but most individuals recorded belonged to one species, the
Caribbean sharp-nose pufferfish (Canthigaster rostrata). As
these are a small-bodied species, habituation effects to the
sounds of shallow OC divers could explain the observed
differences in abundance and biomass between techniques.
If this were the case, it is not clear why similar abundance pat-
terns were not detected in many other reef fish families. For
example, our Lutjanidae results suggest that in the shallows
individuals are not habituated to diver presence, with higher
biomass recorded by CCR than OC, with less difference at
depth (Figure 3C). Tetraodontidae were, however, detected
on the most transects of all fish families tested, and are one
of the most abundant fish detected on Utila across both
shallow reefs and upper-MCEs (Andradi-Brown et al.,
2016b). This high number of transects combined with large
numbers of individuals recorded meant we had greater
power to detect differences in abundance between techniques
and across the depth gradient in Tetraodontidae than
many other fish families we tested. However, our MAD
results for Tetraodontidae run counter to this explanation,
as we found no differences in MAD between OC and CCR,
and MAD declined with increased depth, therefore MCE
Tetraodontidae allowed divers to approach more closely
Fig. 4. Visualization of MAD (A) Depth:Length interactions for Haemulidae, (B) Method:Length interaction for Pomacentridae, (C) Method:Length interaction
for Labridae and (D) Depth:Method interaction for Labridae. Minimum approach distance and fish length and depth have been standardized for the effects of site
and depth/fish length then centred.
8 dominic a. andradi-brown et al.
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than shallow reef ones. This is the reverse pattern expected if
this species was habituated to the presence of OC divers on
shallow reefs. MAD measurements with DOV are most effect-
ive for larger fish species that can be observed with both
cameras in the stereo-video pair in order to measure distance.
DOV has also been shown to be poorer compared with some
other survey techniques at detecting smaller-bodied indivi-
duals such as Tetraodontidae (Andradi-Brown et al., 2016c).
Therefore caution is required when interpreting MAD
values for small-bodied species that do not respond to
divers by swimming away, but instead hide in the reef struc-
ture. If the majority of disturbed fish hide before we observe
them on both cameras then we are unable to record this
high MAD distance, potentially biasing our comparison.
As many in-water diver surveys of shallow-reef communities
are conducted by OC (English et al., 1997), while those on
MCEs are generally conducted by CCR (Bejarano et al., 2014;
Pinheiro et al., 2016), many researchers have been cautious of
making comparisons across the depth gradient between data-
sets collected with different techniques. While caution is
required when analysing data that compounds depth and
dive gear, our results suggest broad comparisons between
shallow reef OC surveys and MCE CCR surveys on Utila are
likely to still give informative patterns relating to biological
changes in fish communities, rather than be primarily driven
by differences between fish detectability caused by dive gear.
Utila however does not have a reef fish spearfishery, and the
MAD changes with depth we observed are unlikely to limit
detectability of fish. However, our MAD results reinforce the
need to account for differing fish behavioural responses
before comparing fish data collected by different dive techni-
ques in areas with spearfisheries. We anticipate that in other
locations where depth-restricted spearfishing occurs (e.g.
Lindfield et al.,2014b), comparisons between different dive
gears will highlight greater dive gear choice effects across the
depth gradient.
Changes in MAD with fish size and depth
We recorded greater MADs for larger individuals in the fam-
ilies Acanthuridae, Labridae, Lutjanidae, Pomacentridae,
Scaridae and Serraninae. These results are consistent with pre-
vious FID studies, which have found larger Acanthuridae and
Scaridae do not allow divers to approach as closely before
fleeing in areas with spearfisheries (Gotanda et al., 2009;
Januchowski-Hartley et al., 2011). In addition, studies of lion-
fish around Utila (which are culled by spearfishers as part of
an invasive species management programme) have found
that larger lionfish react to diver presence at greater distances
than smaller lionfish (Andradi-Brown et al., 2017). However,
spearfishing has been banned around Utila since 2004 for all
species except lionfish (Kramer et al., 2015), making it unlikely
that historical fisheries are driving our observed patterns.
Another explanation is proposed by Gotanda et al.(2009)
based on ecological processes unrelated to fishing pressure:
Generally as reproductive value increases it is predicted that
risk taking should decrease (Clark, 1994). As mortality rates
decline with increased size in marine fish (Sogard, 1997),
larger individuals would therefore be predicted to reduce
risk taking.
Changes in MAD were recorded for six out of eight families
based on the survey depth (Haemulidae, Labridae, Lutjanidae,
Pomacentridae, Scaridae and Tetraodontidae), with in all
cases lower MAD at greater depths. The drivers of this
pattern are not clear, and there could be several possible
explanations. These results do not fit with previous work
in the Bay Islands of Honduras, which suggest that reef
fish can habituate to diver presence (Titus et al., 2015),
and (despite shallow reef culling) that lionfish show no dif-
ference in diver response distance between shallow reefs and
MCEs (Andradi-Brown et al., 2017). With the majority of
diving on Utila limited to shallow reefs, habitation effects
in the shallows would be expected to lead to increasing
MAD with increasing depth. Other possible explanations
include the changing reef structure across the depth gradi-
ent. While data on structural complexity of the reefs at
each depth is not available, previous work at two of our
sites, combined with wider surveys at other sites around
Utila has indicated that hard substrata cover declines with
increased depth (Andradi-Brown et al., 2016b). This
decline in hard substrata, partially caused as the reefs shift
from a spur and groove shallow reef system to a MCE
patch reef system, is also associated with a decline in struc-
tural complexity. Fish on MCE patch reefs may be less likely
to flee from approaching divers, as this would require
moving away from the reef over a large area of open sand,
allowing divers to move closer to them than in the shallows
where a continuous reef exists. Another explanation could
fish to identify diver approaches. However, a study of reef
fish visual acuity across shallow to mesophotic reefs suggests
high plastic adaptive ability of fish visual systems to com-
pensate for lower light levels (Brokovich et al., 2010).
These adaptations were sufficient for a zooplanktivorous
species from the family Pomacentridae to show little
change in foraging behaviour across a shallow reef to
upper-MCE gradient (Brokovich et al., 2010). Natural
changes in reef structure with depth combined with fish
visual adaptive plasticity make it hard to disentangle
effects of depth from those of changing reef structure and
light levels, and require further research.
In addition to a general decline in MAD at increased
depth, in Haemulidae we detected weak Depth:Length inter-
actions. This suggests larger fish are more likely to flee on
MCEs than shallow reefs than would be expected from their
body size alone. Previous work has hypothesized that light
levels may interact with fish length in affecting FIDs
(Januchowski-Hartley et al., 2011), as generally there is an
improvement in fish vision with increasing body size
(Fernald, 1985). This could explain these results, as fish of all
body sizes are more likely to be able to detect divers approach-
ing in shallow sites with high light availability, whereas at
deeper depths with lower light levels it may be harder for
small fish to identify divers approaching than large fish.
This study highlights the importance of considering
changes in fish behaviour when conducting reef fish surveys
across depth gradients. While for many fish families we did
not detect differences in abundance between OC and CCR
surveys, we did identify family level differences in how close
divers could approach individual fish between the methods.
Some differences in approach distance varied with depth,
with the direction of response family specific. This study
highlights the need for reef fish community studies making
comparisons across natural gradients such as depth to be
aware of changes in fish behaviour that may affect their fish
detection results.
dive gear and depth effects on reef fish 9
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We wish to thank Faye-Marie Crooke, Sarah Laverty and
Richard Astley (Coral View Research Center) and Marı
Arteaga and Suriel Duen
˜as (Bay Islands Conservation
Association) for fieldwork support. We also wish to thank
Luke Shepherd, Edd Stockdale and Georgina Wright for
assisting with transect filming. We thank Tom Letessier for
advice on univariate PERMANOVA, and two anonymous
reviewers who helped improve this manuscript.
All authors would like to express thanks to the fieldwork
funders: Royal Geographical Society Ralph Brown
Expedition Award, Zoological Society of London Erasmus
Darwin Barlow Expedition Grants, Operation Wallacea and
the University of Oxford Expeditions Council. DAAB is
jointly funded by a Fisheries Society of the British Isles PhD
studentship and by Operation Wallacea.
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dive gear and depth effects on reef fish 11
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... However, wariness in fish using SVCs has been commonly studied and similar results have been found. For example, Andradi-Brown et al. (2018) found that a juvenile Acanthuridae (9.9 cm length) had a MAD of 2.05 m while a mature individual (23.2 cm long) had a MAD of 2.9 m, demonstrating a 0.85 m difference [70]. Studies on caimans and crocodiles have found similar results, in that wariness is affected by size of the individual [71][72][73]. ...
... The loggerhead sea turtles we observed were composed mostly of subadult and adult size classes, which are inherently larger than the juvenile green sea turtles; therefore, this could explain their increased wariness during our study ( Figure 5). However, additional studies comparing sea turtle behavior at sites frequently visited by humans versus sites that are more secluded would give better insight into how the presence or absence of humans affects sea turtle behavior, similar to past studies conducted for fish species [57,70,76]. ...
... size classes, which are inherently larger than the juvenile green sea turtles; therefore, this could explain their increased wariness during our study ( Figure 5). However, additional studies comparing sea turtle behavior at sites frequently visited by humans versus sites that are more secluded would give better insight into how the presence or absence of humans affects sea turtle behavior, similar to past studies conducted for fish species [57,70,76]. Our SVC approach provides insight into sea turtle behavior and yields a more complex picture into sea turtle behavior than previously recorded. ...
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Our understanding of size-specific sea turtle behavior has lagged due to methodological limitations. However, stereo-video cameras (SVC) are an in-water approach that can link body-size and allow for relatively undisturbed behavioral observations. In this study, we conducted SVC dive surveys at local artificial reefs, piers, and jetties in the northern Gulf of Mexico (nGOM) from May 2019 to August 2021. Using SVCs, we measured sea turtle straight carapace length, documented behaviors, and quantified wariness by assessing minimum approach distance (MAD). In green sea turtles (Chelonia mydas), the observed MAD ranged from 0.72 to 5.99 m (mean 2.10 m ± 1.10 standard deviation (SD), n = 73). For loggerhead sea turtles (Caretta caretta), the MAD ranged between 0.93 and 3.80 m (mean 2.12 m ± 0.99 SD, n = 16). Kemp’s ridley sea turtles (Lepidochelys kempii) were similar to loggerheads, and MAD ranged from 0.78 to 3.63 m (mean 2.35 m ± 0.99 SD, n = 8). We then evaluated what biological factors could impact the MAD observed by species, but we excluded Kemp’s ridleys as the sample size was small. Using a linear mixed model and model selection based on AICc, the top ranked model for both green and loggerhead sea turtles included SCL as the most important factor influencing MAD. MAD did not vary with habitat type for either species. Our results showed that larger individuals, regardless of species, have a greater wariness response, becoming startled at greater distances than smaller individuals. The findings of our study support the use of SVC as an accessible, non-invasive tool to conduct ecologically relevant in-water surveys of sea turtles to link behavioral observations to body size.
... Floating pontoons are featureless themselves, and have a homogenised surface that, depending on the time since deployment, contain attached marine growth that provide fish nursery refugia (Mercader et al., 2017;Selfati et al., 2018), presumably before reaching a size where they leave the safety of the pontoon habitat, to journey more broadly in estuaries and the coastal zone. Previous surveys inspecting fish on pontoons have been completed with divers in the water (Selfati et al., 2018), which could be invasive and prohibit fish from their natural behaviour because of the disturbance caused by the interaction (Andradi-Brown et al., 2018). Advancements with underwater camera and remotely operated vehicles has improved surveys, providing a less invasive survey approach of fish in the environment (Bosch et al., 2017;Sheaves et al., 2016). ...
... While this study provides more evidence that fish utilise pontoon networks (Selfati et al., 2018) and that strong differences exist in fish assemblage between winter and summer which aligns with local data in natural estuaries (Sheaves and Johnston, 2009), our novel remotely operated vessel with underwater camera approach is the first to show that fish adapt to this novel habitat with a behaviour of orienting themselves to the underside of floating pontoons to a substantial extent. This behaviour has not been previously observed on eco-engineered structures, possibly because of the more invasive diver interaction methods used; UAV is increasingly used in coastal areas to provide data from less intrusive sampling approaches (Andradi-Brown et al., 2018;Bosch et al., 2017;Reis-Filho et al., 2020). This ori-entation characteristic was not solely a response by a single or a group of species, but a large proportion of fish community observed here exhibited this behaviour, and from survey to survey, giving rise to anthropomorphisation of marine engineering habitats, where species have simply adapted and use the newly available habitat resource in a positive way. ...
... Underwater video cameras (GoPro Hero3 and Hero4) were deployed at tagged stations (Titus et al., 2015b(Titus et al., , 2017a(Titus et al., , 2017b(Titus et al., , 2019 to minimize the effect of diver presence (Titus et al., 2015a;Andradi-Brown et al., 2018) and increase observation length. Cameras were deployed daily at ∼7:30 am, 10:30 am or 2:00 pm from 20 June to 18 July (Banco Capiro) and 20 July to 7 August (Utila). ...
... Although precautions were taken to minimize the effect of diver presence on fish behaviour, we must consider that diver presence during DOV surveys will likely have had some influence on the data of resident fish assemblages (Watson & Harvey, 2007;Andradi-Brown et al., 2018). Additionally, the distance between the SVS operator and the substratum can make it more difficult to detect smaller and more cryptic fishes among the resident assemblage than when recording clientele at cleaner stations. ...
Cleaning mutualisms are important interactions on coral reefs. Intraspecific variation in cleaning rate and behaviour occurs geographically and is often attributed to local processes. However, our understanding of fine-scale variation is limited, but would allow us to control for geography and region-specific behavioural patterns. Here, we compare the cleaning activity of Pederson's cleaner shrimp ( Ancylomenes pedersoni ) on two neighbouring, yet ecologically dissimilar, reef systems in Honduras: Banco Capiro, an offshore bank close to significant land runoff with high coral cover but a depleted fish population, and an oligotrophic fringing reef around the island of Utila, with lower coral cover but high fish abundance and diversity. The proportion of realized to potential fish clientele was <60% at both sites, and the composition of clientele was neither reflective of the demographics of the resident assemblages at each site nor similar between sites. Parrotfishes represented 13–15% of total fish abundance at both sites yet accounted for >50% (Banco Capiro) and 10% (Utila) of all cleans. Conversely, the schoolmaster snapper ( Lutjanus apodus ) represented ~1% of total fish abundance at both sites yet accounted for 40% (Utila) and 1% (Banco Capiro) of all cleans. After standardizing our cleaning rate data by clientele abundance, we find that clientele at Banco Capiro engage in over four times as many cleaning encounters per hour with A. pedersoni than at Utila. Our study highlights the variable nature of coral reef cleaning interactions and the need to better understand the ecological and environmental drivers of this biogeographic variation.
... Fish length distributions can provide useful indications of fisheries impacts [32]. We calculated the 90th quantile of fish lengths for the combined herbivorous and key fisheries families/subfamilies, providing a simple indicator of the typical size of common larger fish on the reef [33]. See Table S3 for more details. ...
Marine protected areas (MPAs) are considered the go-to tool for marine area-based conservation, having rapidly expanded in many countries, yet an understanding of progress and achievement in implementation remains vastly unexplored. Here, we assess the status and trends of coverage and protection, ecological conditions, marine resource use, human well-being, and management effectiveness of MPAs in the Sunda Banda Seascape (SBS), Indonesia, and provide the first comprehensive overview of marine conservation in the region. The SBS, located in the world's epicenter for marine biodiversity, has 85 designated MPAs (8.1 million ha) as of 2018, accounting for 35% of the national target of 23.4 million ha MPAs by 2020. Most SBS MPAs were recently initiated or established, and their management was strongly influenced by policy and governance changes at the national and provincial levels. SBS MPAs protect more than 30% of mangrove, seagrass, and coral reef habitats in the seascape, and surveys indicate relatively healthy coral reef conditions. Fishing is one of the most common livelihoods in the MPA communities; 90% of fishers use traditional gear types for fishing and destructive fishing is identified as the greatest threat to marine resources in the MPAs. Our findings suggest that balancing the expansion of MPAs with strengthening their management effectiveness and integrating diverse social-ecological local contexts are important aspects to achieve effective, equitable marine conservation.
... Although global in extent, UVCs are limited in depth (usually <20 m) and can potentially underrepresent the diversity and abundance of largebodied fished species (Willis & Babcock 2000;Rojo et al. 2021). Further, some targeted fish display behavioral changes to the diver's presence along gradients of human footprints (Lindfield et al. 2014), including those associated with depth (Andradi-Brown et al. 2018). ...
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Marine fisheries in coastal ecosystems in many areas of the world have historically removed large-bodied individuals, potentially impairing ecosystem functioning and the long-term sustainability of fish populations. Reporting on size-based indicators that link to food-web structure can contribute to ecosystem-based management, but the application of these indicators over large (cross-ecosystem) geographical scales has been limited to either fisheries-dependent catch data or diver-based methods restricted to shallow waters (<20 m) that can misrepresent the abundance of large-bodied fished species. We obtained data on the body-size structure of 82 recreationally or commercially targeted marine demersal teleosts from 2904 deployments of baited remote underwater stereo-video (stereo-BRUV). Sampling was at up to 50 m depth and covered approximately 10,000 km of the continental shelf of Australia. Seascape relief, water depth, and human gravity (i.e., a proxy of human impacts) were the strongest predictors of the probability of occurrence of large fishes and the abundance of fishes above the minimum legal size of capture. No-take marine reserves had a positive effect on the abundance of fishes above legal size, although the effect varied across species groups. In contrast, sublegal fishes were best predicted by gradients in sea surface temperature (mean and variance). In areas of low human impact, large fishes were about three times more likely to be encountered and fishes of legal size were approximately five times more abundant. For conspicuous species groups with contrasting habitat, environmental, and biogeographic affinities, abundance of legal-size fishes typically declined as human impact increased. Our large-scale quantitative analyses highlight the combined importance of seascape complexity, regions with low human footprint, and no-take marine reserves in protecting large-bodied fishes across a broad range of species and ecosystem configurations.
... Examples of field studies of flight initiation distance (FID) of animals in response to human observers, model or actual predators, organized according to features of the prey, the predator, and the environment that influence variation in FID. Category regular open-circuit SCUBA (noisy) as opposed to quiet closed-circuit rebreathers (Lindfield et al. 2014;Andradi-Brown et al. 2018). ...
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Variation in the behaviour of individuals or species, particularly their propensity to avoid or approach human observers, their conveyances (e.g., cars), or their proxy devices (e.g., drones) has been recognized as a source of bias in transect counts. However, there has been little attempt to predict the likelihood or magnitude of such biases. Behavioural ecology provides a rich source of theory to develop a general framework for doing so. For example, if animals perceive observers as predators, then the extensive body of research on responses of prey to their predators may be applied to this issue. Here we survey the literature on flight initiation distance (the distance from a predator or disturbance stimulus at which prey flee) for a variety of taxa to suggest which characteristics of the animal, the observer, and the environment may create negatively biased counts. We also consider factors that might cause prey to approach observers, creating positive bias, and discuss when and why motivation for both approach and avoidance might occur simultaneously and how animals may resolve such trade-offs. Finally, we discuss the potential for predicting the extent of the behaviourally mediated biases that may be expected in transect counts and consider ways of dealing with them.
... Diving spots are commonly characterized by a high diversity of corals and other marine lives. While there is no study about the impact of diving activities on fish communities in diving spots around Misool or Raja Ampat, some research elsewhere has reported that diving activities may affect fish behavior [16] and fish abundance and community structure [17]. ...
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Batbitim marine conservation area (MCA) of Misool, Raja Ampat has been set as an area that is prohibited for fishing activities since 2005. The only activities allowed in that area are tourism and research activities. The difference in the management status between area inside the Batbitim MCA and outside the MCA might affect ecosystem components such as fish and coral reef in the respective area. The present study aims to investigate the ecological status of target fishes in the two areas. Data were collected by using an underwater visual census at 5 sites, in which at each site 3 transects were placed. Collected data are then used to assess ecological indices for the target fishes. It is found that there were 38 species of target fish belonging to 13 families. The diversity index of Shannon was found to be in the range between 0.99 (inside MCA) to 1.67 (outside MCA) and dominance index ranged between 0.26 (outside MCA) and 0.61(inside MCA). The abundance of individual target fish in each location varies between 960 ind ha ⁻¹ (outside MCA) and 9413 ind.ha ⁻¹ (inside MCA). Those results indicate that there is a discrepancy between the ecological status of the target fish at locations inside and outside the MCA.
... Wariness is a sensitive indicator of behavioural changes to fishing pressure (Goetze et al., 2017), whereby fish can become increasingly timid when exposed to fishing and thus stay at a greater distance from observers (Bergseth et al., 2016;Januchowski-Hartley et al., 2015), particularly among larger fish (Andradi-Brown et al., 2017;Benevides et al., 2018;Goetze et al., 2017). In this study, fish wariness for key targeted species outside the marine reserve was limited due to the low abundance or absence of target species. ...
Underwater visual census (UVC) is currently the primary tool used to survey shallow water fish assemblages in the Mediterranean Sea. However, the rapid development of digital technologies, such as underwater video cameras and photogrammetric techniques, are providing new sampling opportunities in marine ecosystems. In this study we compare two non-destructive sampling methods, UVC and diver operated stereo-video systems (stereo-DOVs), to characterize assemblages of targeted fish species. Surveys were undertaken in three zones with different levels of protection from fishing (MR, marine reserve; PP, partially protected reserve; NP, non-protected area) in the Montgrí, Medes Islands and Baix Ter Natural Park (NW Mediterranean Sea). There were no statistically significant differences between stereo-DOVs and UVC surveys in terms of species richness, relative abundance and biomass of targeted fish species across the three protection levels. Both methodologies found statistically significant differences between areas closed to fishing (MR) and areas where fishing is regulated (PP) or allowed (NP), whereby higher fish biomass, density, species richness and larger sizes were found in MR surveys. Length estimates of fish were comparable between methods albeit with some species-specific differences in the size-distribution, and a tendency for UVC to overestimate the size of larger fishes. Our results suggest that stereo-DOVs provide an equivalent and complementary technique to UVC to survey assemblages of targeted fish species. In addition to structural data (e.g., biomass, abundance, and length) recorded by UVC, stereo-DOVs provide a permanent visual record of a fish community and enable additional factors such as fish behaviour, precise distance and length, and benthic composition to be quantified. Stereo-DOVs may, therefore, facilitate the continuity and expansion of long-term monitoring programs of fish assemblages in the Mediterranean Sea.
... Despite the increase in the literature on fish assemblage structure in deep and/or mesophotic reefs, little is known about behavioral change in relation to the depth gradient (but see Andradi-Brown et al. 2017). Our study showed that the relationship between depth and antipredator behavior varies by species. ...
The reduction or loss of antipredator behavior is expected for animals isolated on islands, but most of studies have focused on terrestrial rather than aquatic species. Two different processes may modulate fish antipredator behavior in the waters off remote archipelagos. First, places with low human density may have reduced fishing pressure. Second, due to their remoteness and lack of other human impacts, these isolated places may have more intact fish assemblages. We investigated antipredator behavior off the smallest archipelago in the Equatorial Atlantic not subjected to regular spearfishing by quantifying flight initiation distance (FID)-the distance a diver can approach a fish before it flees. Our goal was to identify natural patterns of risk assessment in reef fish. We focused on five common species and investigated how body size, group size and depth influence on the FID. In contrast to previous studies, we found that Caranx lugubris had significantly negative relationships between body size and FID, whereas Kyphosus sectatrix showed an opposite relationship. Finally, only a single species (Melichthys niger) had a significant relationship between depth and FID. Given we found small FID for all species studied, we suggest that much of our understanding of fish antipredator behavior studied using FID may reflect the somewhat ubiquitous impact of humans on species' risk assessments and not reflect patterns seen in areas with very low human density/disturbance. Our results suggest that fish anti-predator behavior may be a metric of human impacts.
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An environmental DNA (eDNA) metabarcoding approach has been widely used for biodiversity monitoring of fishes, although it has rarely been applied to tropical and subtropical aquatic ecosystems, where species diversity is remarkably high. This study examined the extent to which species richness can be estimated in a small coral reef lagoon (1,500 × 900 m) near Okinawa Island, southern Japan, where the surrounding waters are likely to harbor more than 1,500 species of fish. During 2015–2017, a total of 16 capture‐based surveys were conducted to create a faunal list of fish species, followed by eDNA metabarcoding based on seawater samples taken from 11 sites in the lagoon on a day in May 2019. We also tested whether eDNA metabarcoding could detect differences between adjacent fish communities inhabiting the offshore reef edge and shore‐side seagrass beds within the lagoon. A total of 217 fish species were confirmed by the capture‐based samplings, while 291 fish species were detected by eDNA metabarcoding, identifying a total of 410 species distributed across 119 families and 193 genera. Of these 410 species, only 96 (24% of the total) were commonly identified by both methods, indicating that capture‐based surveys failed to collect a number of species detected by eDNA metabarcoding. Interestingly, two different approaches to estimate species richness based on eDNA data yielded values close to the 410 species, including one that suggested an additional three or more eDNA surveys from 11 sites (36 samples) would detect 90% of the 410 species. In addition, nonmetric multidimensional scaling for fish assemblages clearly distinguished between the fish communities of the offshore reef edge and those of the shore‐side seagrass beds. This study demonstrates that an eDNA metabarcoding approach is useful for estimating species richness and detection of habitat segregation even in ecosystems with remarkably high species diversity.
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Invasive lionfish (Pterois volitans and P. miles) have spread widely across the western Atlantic and are recognized as a major threat to native marine biodiversity. Although lionfish inhabit both shallow reefs and mesophotic coral ecosystems (MCEs; reefs from 30 to 150 m depth), the primary management response implemented by many countries has been diver-led culling limited to reefs less than 30 m. However, many reef fish undergo ontogenetic migrations, with the largest and therefore most fecund individuals found at greatest depths. Here, we study lionfish density, body size, maturity and dietary patterns across the depth gradient from the surface down to 85 m on heavily culled reefs around Utila, Honduras. We found lionfish at increased densities, body size and weight on MCEs compared with shallow reefs, with MCEs also containing the greatest proportion of actively spawning females, while shallow reefs contained the greatest proportion of immature lionfish. We then compared lionfish behaviour in response to divers on shallow culled and mesophotic unculled Utilan reefs, and on shallow unculled reefs in Tela Bay, on the Honduran mainland. We found that mesophotic lionfish exhibited high alert distances, consistent with individuals previously exposed to culling despite being below the depth limits of removal. In addition, when examining stomach content, we found that fish were the major component of lionfish diets across the depth gradient. Importantly, our results suggest that despite adjacent shallow culling, MCEs retain substantial lionfish populations that may be disproportionately contributing towards continued lionfish recruitment onto the shallow reefs of Utila, potentially undermining current culling-based management.
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Fish surveys form the backbone of reef monitoring and management initiatives throughout the tropics, and understanding patterns in biases between techniques is crucial if outputs are to address key objectives optimally. Often biases are not consistent across natural environmental gradients such as depth, leading to uncertainty in interpretation of results. Recently there has been much interest in mesophotic reefs (reefs from 30-150 m depth) as refuge habitats from fishing pressure, leading to many comparisons of reef fish communities over depth gradients. Here we compare fish communities using stereo-video footage recorded via baited remote underwater video (BRUV) and diver-operated video (DOV) systems on shallow and mesophotic reefs in the Mesoamerican Barrier Reef, Caribbean. We show inconsistent responses across families, species and trophic groups between methods across the depth gradient. Fish species and family richness were higher using BRUV at both depth ranges, suggesting that BRUV is more appropriate for recording all components of the fish community. Fish length distributions were not different between methods on shallow reefs, yet BRUV recorded more small fish on mesophotic reefs. However, DOV consistently recorded greater relative fish community biomass of herbivores, suggesting that studies focusing on herbivores should consider using DOV. Our results highlight the importance of considering what component of reef fish community researchers and managers are most interested in surveying when deciding which survey technique to use across natural gradients such as depth. © 2016 Andradi-Brown et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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Visual survey by divers using open-circuit (OC) SCUBA is the most widely used approach to survey coral reef fishes. Therefore, it is important to quantify sources of bias in OC surveys, such as the possibility that avoidance of OC divers by fishes can lead to undercounting in areas where targeted species have come to associate divers with a risk of being speared. One potential way to reduce diver avoidance is to utilize closed circuit rebreathers (CCRs), which do not produce the noise and bubbles that are a major source of disturbance associated with OC diving. For this study, we conducted 66 paired OC and CCR fish surveys in the Main Hawaiian Islands at locations with relatively high, moderate, and light fishing pressure. We found no significant differences in biomass estimates between OC and CCR surveys when data were pooled across all sites, however there were differences at the most heavily fished location, Oahu. There, biomass estimates from OC divers were significantly lower for several targeted fish groups, including surgeonfishes, targeted wrasses, and snappers, as well as for all targeted fishes combined, with mean OC biomass between 32 and 68% of mean CCR biomass. There were no clear differences between OC and CCR biomass estimates for these groups at sites with moderate or low fishing pressure, or at any location for other targeted fish groups, including groupers, parrotfishes, and goatfishes. Bias associated with avoidance of OC divers at heavily fished locations could be substantially reduced, or at least calibrated for, by utilization of CCR. In addition to being affected by fishing pressure, the extent to which avoidance of OC divers is problematic for visual surveys varies greatly among taxa, and is likely to be highly influenced by the survey methodology and dimensions used.
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Mesophotic coral ecosystems (MCEs; reefs 30-150m depth) are of increased research interest because of their potential role as depth refuges from many shallow reef threats. Yet few studies have identified patterns in fish species composition and trophic group structure between MCEs and their shallow counterparts. Here we explore reef fish species and bio-mass distributions across shallow to upper-MCE Caribbean reef gradients (5-40m) around Utila, Honduras, using a diver-operated stereo-video system. Broadly, we found reef fish species richness, abundance and biomass declining with depth. At the trophic group level we identified declines in herbivores (both total and relative community biomass) with depth, mostly driven by declines in parrotfish (Scaridae). Piscivores increased as a proportion of the community with increased depth while, in contrast to previous studies, we found no change in relative planktivorous reef fish biomass across the depth gradient. In addition, we also found evidence of ontogenetic migrations in the blue tang (Acanthurus coeruleus), striped parrotfish (Scarus iserti), blue chromis (Chromis cyanea), creole wrasse (Clepticus parrae), bluehead wrasse (Thalassoma bifasciatum) and yellowtail snapper (Ocyurus chry-surus), with a higher proportion of larger individuals at mesophotic and near-mesophotic depths than on shallow reefs. Our results highlight the importance of using biomass measures when considering fish community changes across depth gradients, with biomass generating different results to simple abundance counts.
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Despite more than 60 yr of coral reef research using scuba diving, mesophotic coral ecosystems (MCEs) between 30 and 150 m depth remain largely unknown. This study represents the first underwater visual census of reef fish communities in the Greater Caribbean on MCEs at depths up to 80 m in Bermuda and 130 m in Curaçao. Sampling was performed using mixed-gas closed-circuit rebreathers. Quantitative data on reef fish communities were obtained for four habitats: coral reefs (45–80 m), rhodolith beds (45–80 m), ledges (85–130 m) and walls (85–130 m). A total of 38 species were recorded in Bermuda and 66 in Curaçao. Mesophotic reef fish communities varied significantly between the two localities. MCEs in Bermuda had lower richness and abundance, but higher biomass than those in Curaçao. Richness, abundance and biomass increased with depth in Bermuda, but decreased in Curaçao. A high turnover of species was found among depth strata and between Bermuda and other Caribbean upper MCEs (45–80 m), indicating that depth was an important driver of community structure at all localities. However, local and evolutionary factors (habitat and endemism) are likely the main factors shaping communities in isolated locations such as Bermuda. High fishing pressure is evident in both localities, as total biomass of apex predators was generally low, and thus may be driving a “refugia” scenario in Bermuda, as the abundance and biomass of macro-carnivores increased with depth and distance from the coast.
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Coral reefs are subjected to unprecedented levels of disturbance with population growth and climate change combining to reduce standing coral cover and stocks of reef fishes. Most of the damage is concentrated in shallow waters (<30 m deep) where humans can comfortably operate and where physical disturbances are most disruptive to marine organisms. Yet coral reefs can extend to depths exceeding 100 m, potentially offering refuge from the threats facing shallower reefs. We deployed baited remote underwater stereo-video systems (stereo-BRUVs) at depths of 10–90 m around the southern Mariana Islands to investigate whether fish species targeted by fishing in the shallows may be accruing benefits from being at depth. We show that biomass, abundance and species richness of fishery-targeted species increased from shallow reef areas to a depth of 60 m, whereas at greater depths, a lack of live coral habitat corresponded to lower numbers of fish. The majority of targeted species were found to have distributions that ranged from shallow depths (10 m) to depths of at least 70 m, emphasising that habitat, not depth, is the limiting factor in their vertical distribution. While the gradient of abundance and biomass versus depth was steepest for predatory species, the first species usually targeted by fishing, we also found that fishery-targeted herbivores prevailed in similar biomass and species richness to 60 m. Compared to shallow marine protected areas, there was clearly greater biomass of fishery-targeted species accrued in mesophotic depths. Particularly some species typically harvested by depth-limited fishing methods (e.g., spearfishing), such as the endangered humphead wrasse Cheilinus undulatus, were found in greater abundance on deeper reefs. We conclude that mesophotic depths provide essential fish habitat and refuge for fishery-targeted species, representing crucial zones for fishery management and research into the resilience of disturbed coral reef ecosystems.