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Interference competition can exclude subordinate species from habitats and alter community structure. An example is the predatory release of mesopredators resulting in changes in their prey and competitor community. Baited, remote, underwater videography is a powerful census tool that can be biased if dominance excludes species from view. At interspecific encounters during BRUV deployments in the Adriatic Sea, Croatia, no such general bias occurred. BRUVS detected few negative co - occurrences for species pairs and they rarely interfered at opportunities near the bait. Most species pairs positively co -occurred and were unique for each habitat type, which indicates that habitat heterogeneity plays an important role in driving fish biodiversity. This research was supported by the Croatian Science Foundation under the project COREBIO (3107).

The European eel, Anguilla anguilla, is critically endangered and according to European council regulations member states need to establish stock recovery measures. Such plans must estimate eel escapement to the sea, thus life stage and habitat-based assessment of coastal eels is crucial, and considering their red-listed status, non-invasive assessment methods are desirable. Within the scope of a pilot eel monitoring, the spatial and temporal distribution of eel have been investigated in the shallow benthos of the Central Croatian Adriatic. Using geo-referenced and video supported SCUBA visual census, transects of up to 1000+ m were followed during the day and then with DGPS sub-meter accuracy re-navigated at various night times. Two bays were investigated at various study sites per bay and over complete moon cycles in the spring and fall of 2009/10 and in the spring of 2011. Eel presence and habitat occupation at the instant of observation were recorded along with their exact geographical location. This permitted the quantification of eel densities and habitat preferences at various scales. Significant positive associations with dense seagrass patches were discovered for the eel, and such habitat preference explained most of the eels’ large scale distribution. Furthermore eel abundance and activity showed significant temporal variation, as they peaked several hours after sunset, at quarter moons, and in the fall. Due to the direct encounter by the observer, additional data, including eel size and behavior are gathered. A future addition of stereo imaging of the censused eels is expected to allow for the identification of life stages (yellow vs. silver eel). The applied method is non-invasive, time and cost effective, combines eel census with simultaneous monitoring of the eels’ benthic environment, can collect data from large areas at various scales, and allows for frequent revisits of monitored locations with a high level of spatial precision. As a future EU member Croatia soon needs to report coastal eel stock assessments and a spatial expansion of the ongoing pilot monitoring scheme with added non-sacrificial life stage recognition may allow for the required estimates of eel escapement to the sea.

Benthic communities are dynamic systems that respond constantly due to natural and anthropogenic impacts. Permanent monitoring programs are necessary for effective management of protected and threatened marine areas. The aim of this study was to use the original method of underwater geo-referenced videography to record the baseline condition of the benthic habitats in the shallow infralittoral of Kornati National Park, with special focus on Posidonia oceanica seagrass meadows. The substrate type, depth, density and taxa of aquatic vegetation and presence of ecological elements was recorded for every second of the video. We recorded 52 geo-referenced video-transects, with the total length of 28.4 km, of which Posidonia covered 14.8 km. The median depth of the highest density Posidonia was 12 m, density decreased with increasing depth, and we saw no Posidonia deeper than 32 m. The density of algal cover also decreased with depth, while the settlements of sparse Cymodocea and loose algae occurred at lower depths than where algae or Posidonia dominated. The median depth of the highest density algae and sparse Cymodocea settlements was about 9 m. The median depth of pen shell (Pinna nobilis) was 11 m. Our results confirmed that underwater geo-referenced videography can provide high-quality data (spatial and visual), which can be quickly collected and quantitatively analyzed, and these data can be collected entirely non-destructively. Since the data are geo-referenced it is possible to return to the same location at any time and re-record the same transect, and quantify any change in habitat with known statistical precision. Its low cost, rapid and non-destructive execution and statistical rigor makes this method highly suitable for monitoring benthic habitats.

Shallow and enclosed bays are prone to experience sedimentation resulting in decreases in slope and overall depth. Seagrass beds are known to foster sedimentation which then either (i) benefits seagrass growth, as the bed expands onto new raised and sufficiently illuminated ground or (ii) hampers it, as fast sedimentation may burry newly growing shoots or interfere with seed germination. The shallow benthos of the westernmost Novigrad Sea, an enclosed bay in the Central Croatian Adriatic, has been monitored by means of videographed, DGPS and depth tracked transects positioned perpendicular to the shore and spanning depths from 0 to maximal 6 m, which is larger then 5.2 m, the maximum depth limit for seagrass in the area. Transects were repeatedly followed and videographed with sub-meter accuracy and subsequent video analyses identified each second (approximately 0.3 transect meters) as either seagrass, rock, algae, or unconsolidated sediment. From June 2007 to June 2008 almost all transects decreased in benthic slope and simultaneously experienced an overall gain in seagrass coverage averaging 40 linear transect meters and equaling an average yearly gain of 22% per transect. We identified most of the seagrass gain as a transition from unconsolidated sediment at various depths but mostly at the deep ends of transects where new shallow ground allowed for additional seagrass growth. Future research will include the continuation of the video monitoring as well as methods to estimate sedimentation rates, and experiments to identify interactions between seagrass growth and sedimentation events.