Depth Preferences Research Articles

Decommissioning impacts on biotic assemblages associated with shell mounds beneath southern California offshore oil and gas platforms

The decommissioning of southern California offshore oil and gas platforms will create major economic, engineering, and environmental challenges in the next decade. Platform jackets, conductors, and shell mounds often host a diverse and productive marine community, and among the myriad considerations associated with decommissioning planning, platform operators and federal and state regulatory agencies will consider the ecological value of existing underwater structures as artificial reefs. In the event of partial removal of platform structure, fish assemblages on decommissioned platforms may remain unchanged in areas where structure is left intact. However, on the seafloor beneath the platforms, a mound of debris often called the shell mound will likely change over time if the supply of falling mussels and other organisms from the productive surface part of the structure is removed. In this study, we review shell mound research relevant to decommissioning, including mound formation, contaminant loads, associated biological communities, and transitions following the removal of platform structures at four sites. To address the gap in knowledge of shell mound fish community structure, we used manned submersible and remotely operated vehicle surveys from 1997 to 2013 to estimate the biomass, density, species composition and similarity between shell mounds at 22 southern California platforms. We found a wide range of variability in fish density and shell mound areal extent. Species composition also varied among sites, with three significant community clusters primarily distinguished by species depth preferences. These results will help inform a comprehensive net environmental benefit analysis of southern California offshore platform decommissioning alternatives.

The effect of bottom trawl fishery on biomass variations of demersal chondrichthyes in the eastern Mediterranean

Chondrichthyan species are considered highly susceptible to fishing activities and as “indicator species” for overfishing and ecosystem stability. In European waters, chondrichthyes have become a focus of conservation actions due to the abundance declines of several species.In the present study, fishery independent data obtained from the Mediterranean International Trawl Surveys (MEDITS) in the Aegean and E. Ionian Seas (eastern Mediterranean) were coupled with bottom trawl fishing effort data obtained from vessel monitoring systems (VMS) in order to evaluate the effect of fishing on biomass variations of demersal chondrichthyan species. The study focused on commonly captured species such as Raja clavata, Galeus melastomus, Scyliorhinus canicula and Squalus spp, as well as to the total chondrichthyan biomass. The effect of fishing effort on biomass indices, expressed in terms of kg per square km of swept area, was examined by means of Generalized Additive Model techniques, using also the sampling position, year and depth as co-variates.Overall findings suggested a biomass decline of chondrichthyan species with increasing fishing effort. Nevertheless, it was also found that certain species (e.g G. melastomus in both areas and S. canicula in the Aegean Sea) can withstand fishing pressure and it is likely that discard survival rates, depth preferences and fisheries exploitation patterns, are the main drivers explaining variations regarding the impact of fishing on the different stocks.

Differences in stress reactivity between zebrafish with alternative stress coping styles.

Animals experience stress in a variety of contexts and the behavioural and neuroendocrine responses to stress can vary among conspecifics. The responses across stressors often covary within an individual and are consistently different between individuals, which represent distinct stress coping styles (e.g. proactive and reactive). While studies have identified differences in peak glucocorticoid levels, less is known about how cortisol levels differ between stress coping styles at other time points of the glucocorticoid stress response. Here we quantified whole-body cortisol levels and stress-related behaviours (e.g. depth preference, movement) at time points representing the rise and recovery periods of the stress response in zebrafish lines selectively bred to display the proactive and reactive coping style. We found that cortisol levels and stress behaviours are significantly different between the lines, sexes and time points. Further, individuals from the reactive line showed significantly higher cortisol levels during the rising phase of the stress response compared with those from the proactive line. We also observed a significant correlation between individual variation of cortisol levels and depth preference but only in the reactive line. Our results show that differences in cortisol levels between the alternative stress coping styles extend to the rising phase of the endocrine stress response and that cortisol levels may explain variation in depth preferences in the reactive line. Differences in the timing and duration of cortisol levels may influence immediate behavioural displays and longer lasting neuromolecular mechanisms that modulate future responses.

Category-selective areas in human visual cortex exhibit preferences for stimulus depth

Multiple regions in the human brain are dedicated to accomplish the feat of object recognition; yet our brains must also compute the 2D and 3D locations of the objects we encounter in order to make sense of our visual environments. A number of studies have explored how various object category-selective regions are sensitive to and have preferences for specific 2D spatial locations in addition to processing their preferred-stimulus categories, but there is no survey of how these regions respond to depth information. In a blocked functional MRI experiment, subjects viewed a series of category-specific (i.e., faces, objects, scenes) and unspecific (e.g., random moving dots) stimuli with red/green anaglyph glasses. Critically, these stimuli were presented at different depth planes such that they appeared in front of, behind, or at the same (i.e., middle) depth plane as the fixation point (Experiment 1) or simultaneously in front of and behind fixation (i.e., mixed depth; Experiment 2). Comparisons of mean response magnitudes between back, middle, and front depth planes reveal that face and object regions OFA and LOC exhibit a preference for front depths, and motion area MT+ exhibits a strong linear preference for front, followed by middle, followed by back depth planes. In contrast, scene-selective regions PPA and OPA prefer front and/or back depth planes (relative to middle). Moreover, the occipital place area demonstrates a strong preference for “mixed” depth above and beyond back alone, raising potential implications about its particular role in scene perception. Crucially, the observed depth preferences in nearly all areas were evoked irrespective of the semantic stimulus category being viewed. These results reveal that the object category-selective regions may play a role in processing or incorporating depth information that is orthogonal to their primary processing of object category information.

Cross-shelf habitat shifts by red snapper (Lutjanus campechanus) in the Gulf of Mexico.

Habitat shifts that occur during the life cycles of marine fishes influence population connectivity and structure. A generalized additive modeling approach was used to characterize relationships between environmental variables and the relative abundance of red snapper Lutjanus campechanus over unconsolidated substrate on the continental shelf (<150 m) of the U.S. Gulf of Mexico (GoM) at three different life stages: juvenile (age-0, <125 mm FL), sub-adult (age-1-2, 125–300 mm FL), and adult (age-2+, >300 mm FL). Fisheries independent data (2008–2014) were used to develop separate models for both the eastern and western GoM, and final models were used to predict the relative availability of suitable habitat for each life stage across the two regions. Predictor variables included in final models varied by age class and region, with depth, dissolved oxygen, longitude, and distance to artificial structure common to most models. Depth was among the most influential variables in all models, and preferred depth increased with increasing size/age. Regional differences in fish-habitat relationships were also observed, as relative abundance of larger red snapper over unconsolidated substrates was more closely linked to artificial structure in the eastern GoM. The location of predicted high quality habitat for juvenile red snapper was greatest on the inner Texas shelf and a smaller area east of the Mississippi River Delta, suggesting these two areas may represent important nursery grounds for the respective regions. Clear ontogenetic shifts in the spatial distribution of predicted high quality habitat were evident in both the eastern (expansion from west to east with age) and western (shift from inshore to offshore) GoM. Given the unique population dynamics between the eastern and western GoM, improving our understanding of spatial and temporal variability in habitat quality may be important to maintaining connectivity between juvenile and adult habitats, and may enhance recovery and management of red snapper stocks in the GoM.

On Intraspecific and Interspecific Variation in Teleost Scleral Ossification.

Scleral ossicles are bony elements found along the eyes of many fishes, amphibians, and reptiles. These bones provide a superficial layer of support to the eye and may facilitate visual acuity. Previous research has shown that scleral ossicle diversity is generally limited among teleosts, but that scleral ossicles have been lost numerous times among teleosts inhabiting benthopelagic habitats (Franz-Odendaal. Anat Rec 291 (2008) 161-168). In this study, we further investigate these patterns of intraspecific and interspecific variation by examining eyes from multiple individuals of 10 riverine teleosts native to Kentucky as well as one population of the Mexican blind cavefish, Astyanax mexicanus, and by re-analyzing a quantitative database of scleral ossicle number and depth preference from over 100 teleosts using newly resolved teleost phylogenies. Consistent with the limited diversity of most teleost families, we find that intraspecific variation in scleral ossicle number and size is virtually nonexistent among the species sampled, although we do find evidence of additional interspecific variation among the Cyprinodontiformes, as well as dramatic intrapopulation variation among cavefish from Chica Cave. Although our data replicates the negative relationship between scleral ossicle number and the depth preference previously found among teleosts (Franz-Odendaal. Anat Rec 291 (2008) 161-168), even when accounting for phylogenetic relationships, our results further reveal that this relationship is relatively weak. We conclude that further sampling may reveal additional interspecific and even intraspecific variation among some groups of teleosts, and that depth could serve as a proxy for other life history traits that more directly influence teleost scleral ossicle diversity such as prey-capture strategies. Anat Rec, 302:1238-1249, 2019. © 2019 Wiley Periodicals, Inc.

Spatio-temporal dynamics and behavioural ecology of a “demersal” fish population as detected using research survey pelagic trawl catches: the Eastern Baltic Sea cod (Gadus morhua)

Abstract Cod is usually monitored for scientific purposes using bottom trawl surveys, although its regular pelagic occurrence is well documented. Here we analysed, using Generalized Additive Models, the spatio-temporal changes in the Eastern Baltic cod adult population using pelagic catches from an acoustic survey covering 37 years and the whole central Baltic Sea. Our analysis shows that in the northern areas cod catch per unit effort (CPUE, kg h-1) was high in the early 1980s whereas it dropped and remained very low thereafter. Conversely, in the southernmost area CPUE largely oscillated after the early 1990s. Our model was able to capture key ecological features of the Baltic cod such as preferred depth of occurrence and response to hypoxic conditions. The model also revealed a clear daily cycle of CPUEs, indicating diel vertical migrations at the population level. The temporal trends of pelagic CPUEs generally followed those from the bottom trawl surveys, although differences were observed especially in the recent years with a relative decline in the cod occurring in the pelagic waters. Our results point to the great potential of acoustic survey trawl catches to complement bottom trawl surveys for investigating the spatio-temporal population dynamics and behaviour of the Baltic cod.

How “The Blob” affected groundfish distributions in the Gulf of Alaska

AbstractWe investigated the distributional shifts of groundfish in response to anomalous ocean conditions, particularly the recent anomalously warm period (2014–2016; “The Blob”), based on data from ten Gulf of Alaska bottom trawl surveys conducted by the Alaska Fisheries Science Center during 1996–2015. Six groundfish species were considered: Pacific cod (Gadus macrocephalus), arrowtooth flounder (Atheresthes stomias), walleye pollock (Gadus chalcogrammus), Pacific ocean perch (Sebastes alutus), northern rock sole (Lepidopsetta polyxystra), and southern rock sole (Lepidopsetta bilineata). Ontogenetic differences were examined by dividing data for each fish species into size classes. Our study demonstrated that after accounting for size‐specific depth preferences, the spatial responses of groundfish to anomalous ocean conditions differed by species and foraging guild in the central Gulf of Alaska. Pacific cod and arrowtooth flounder showed similar responses to ocean warming, but different responses to cooling. In general, Pacific cod moved to deeper depths in warmer years and moved to shallower depths in colder years. Arrowtooth flounder also moved deeper in warmer years. However, in colder years, large arrowtooth flounder (&gt;40 cm) shifted toward shallower depths while smaller‐sized fish shifted toward deeper depths. In warmer years, large pollock (&gt;30 cm) moved to deeper waters while smaller pollock (10–20 cm) moved to shallower waters. Pacific ocean perch exhibited an opposite response to thermal changes in habitat compared with Pacific cod and arrowtooth flounder. They moved deeper in colder years, but there was no clear change in depth as a function of size in response to warmer habitat.

Spatial contraction of demersal fish populations in a large marine ecosystem

AbstractAimThe interdependencies between trophic interactions, environmental factors and anthropogenic forcing determine how species distributions change over time. Large changes in species distributions have occurred as a result of climate change. The objective of this study was to analyse how the spatial distribution of cod and flounder has changed in the Baltic Sea during the past four decades characterized by large hydrological changes.LocationBaltic SeaTaxonCod (Gadus morhua) and flounder (Platichthys flesus).MethodsCatch per unit of effort (CPUE) data for adult and juvenile cod and for adult flounder were modelled using Delta‐Generalized additive models including environmental and geographical variables between 1979 and 2016. From the annual CPUE predictions for each species, yearly distribution maps and depth distribution curves were obtained. Mean depth and the depth range were estimated to provide an indication on preferred depth and habitat occupancy.ResultsAdult and juvenile cod showed a contraction in their distribution in the southern areas of the Baltic Sea. Flounder, instead, showed an expansion in its distribution with an increase in abundance in the northern areas. The depth distributions showed a progressive shift of the mean depth of occurrence towards shallower waters for adult cod and flounder and towards deeper waters for juvenile cod, as well as a contraction of the species depth ranges, evident mainly from the late 1980s.Main conclusionsOur study illustrates large changes in the spatial distribution of cod and flounder in the Baltic Sea. The changes in depth distribution occurred from the late 1980s are probably due to a combination of expanded areas of hypoxia in deep waters and an increase in predation risk in shallow waters. The net effect of these changes is an increased spatial overlap between life stages and species, which may amplify cod cannibalism and the interaction strength between cod and flounder.

Egg Extraction Methods for Bagrada hilaris (Heteroptera: Pentatomidae) with Notes on Ovipositional Preferences

The ovipositional preference of Bagrada hilaris (Burmeister) was evaluated, and egg extraction methods from soil were compared. In a choice test between soil, plant material, and exposed plastic surfaces, females laid eggs only in the soil. Significantly more eggs were deposited in dry soil than in most soil. A significant preference for ovipositional depth within the soil was found, with the greatest proportion of eggs laid in the upper one-third (i.e., 0–0.4 cm) of the soil. Two egg extraction methods (i.e., wash and flotation) were compared for effectiveness, sampling time, and cost of setup. The wash method consisted of two types (wash methods 1 and 2, with or without a paint strainer, respectively), whereas the flotation method consisted of several solutions. The wash method (both types combined) was more effective in egg extraction than the flotation method from both small and large volume samples, but the two types differed from each other; method 1 had a higher recovery rate than method 2 for small soil volumes but a lower recovery rate than method 2 for large volumes. Total sampling time was shorter for the wash method than the flotation method and less expensive.

A preliminary study on the distribution of spiny lobster (Panulirus spp.) in Labuan Island, Malaysia

Limited info regarding on the global distribution and fisheries data on the spiny lobster Panulirus spp. had placed the IUCN redlist in a dilemmatic position in determining the concrete status for proper conservation. The present survey was carried out in the western region of the main island which consists of seven random stations along the west coast of Labuan during day dive. An extreme low density of the spiny lobsters from the genus Panulirus was determined throughout the present study. Only four individuals of two spiny lobsters species were recorded. Two individuals of P. versicolor were observed in station 2 while the other two (P. ornatus) were spotted in station 4. Low numbers of the Panulirus spp. were spotted during the present preliminary survey was probably due to day dive sampling. Spiny lobster from the genus Panulirus tend to have a wide range of depth preference. Two adults P. versicolor were spotted in least turbid environment. In the previous studies, Panulirus spp. such as P. homarus, P. ornatus, and P. polyphagus tend to prefer slightly turbid environment. The results of the present preliminary daytime survey indicated only two species of Panulirus spp. recorded with two individual numbers per species in Labuan coastal waters. However, due to the little numbers of catchment during the preliminary survey, further studies need to be carried out to provide more concrete evidences on the depth preference of Panulirus spp. in Malaysian waters.

Immigration and extirpation selectivity patterns of brachiopods and bivalves across a Carboniferous glacial to non-glacial transition (Pennsylvanian, central western Argentina) and their influence in building the biotic bathymetric gradient

The growth and demise of glaciers during the Late Paleozoic ice age had profound consequences on the marine biota. In central western Argentina the transition from glacial to non-glacial interval during the Pennsylvanian, caused a major regional turnover which remodeled the bathymetric diversity gradient. In order to understand the nature of the environmental changes related to such turnover event, we compared selectivity patterns in extirpation and immigration of bivalves and brachiopods using logistic regression. We tested for a wide range of potential drivers of extirpation and immigration, namely body size, geographic range size, environmental breadth, preferred depth, metabolic rates, infaunalism and taxonomic identity. While extirpation preferentially hit brachiopods and genera having smaller geographic ranges and smaller body sizes, immigration was related to genera with preference for deeper environment and small geographic ranges.Extirpation selectivity was not uniform along the bathymetric gradient. Both geographic range and body size selectivity are stronger towards deeper environments. Each clade, on the other hand, showed idiosyncratic responses to climate change. While brachiopods show extirpation selectivity for smaller geographic ranges, small sized bivalves were preferentially extirpated. These contrasting selectivity regimes, however, could be reflecting different faunal responses to the same environmental changing factor, namely higher thermal amplitude. Immigration in both brachiopods and bivalves was related to preference to deep environments, confirming previous results. However, brachiopods also showed preferential immigration of small geographic range genera, which is puzzling given the extinction selectivity related to the glacial non-glacial transition. Possible explanations point to a brief time span in which an increase in thermal amplitude occurred. The rapid climate changes, particularly related to a rise in maximum temperatures, lead the ecological response of the benthic faunas. Differences in extinction and immigration selectivity along the bathymetric gradient, suggest that the turnover event also remodeled the bathymetric gradient in body size and biogeographic structure and brachiopod:bivalve proportions. Such changes were driven by extirpation, contrasting with the remodeling of the bathymetric diversity gradient that was driven by immigration. Our results, therefore, highlight that immigration and extirpation can shape different aspects of the bathymetric biotic gradient.

The tapeworm Ligula intestinalis alters the behavior of the fish intermediate host Engraulicypris sardella, but only after it has become infective to the final host

Ligula intestinalis is a tapeworm using copepods and cyprinid fish as intermediate hosts and fish-eating birds as final hosts. Since some parasites can increase their own fitness by manipulating the behavior of the intermediate host, we explored if this parasite affected predator avoidance, swimming activity and depth preference of the fish intermediate host, Engraulicypris sardella. We found that when L. intestinalis had reached a developmental stage that is able to establish in the bird host, it had a significant impact on E. sardella behavior, while the tapeworm that was not fully developed had little effect and fish hosts showed a behavior more similar to uninfected fish. These results are discussed with respect to two different processes: the manipulation hypothesis and the energy drain hypothesis.

TCT-494 Influence of High Device Positioning on Outcomes after Transcatheter Aortic Valve Replacement with Contemporary Self-Expanding Valves

The preferred depth of implant for self-expanding transcatheter aortic valve replacement (TAVR) has become increasingly high. However, the impact on outcomes of contemporary self-expanding valves (SEV), which are both retrievable and repositionable, has been poorly elucidated. We sought to study the

Co-occurrence of the charophyte Lychnothamnus barbatus with higher trophy submerged macrophyte indicators

Lychnothamnus barbatus, one of the rarest and endangered charophytes world-wide, is a bioindicator of low water trophy. This study was carried out in a moderately fertile water body hosting a large population of L. barbatus (Lake Kuźnickie, Poland). In this lake, L. barbatus co-occurs in submerged vegetation with higher trophy indicators, aquatic angiosperms Ceratophyllum demersum and Myriophyllum spicatum and the common charophyte Nitellopsis obtusa. The study aim was to investigate the depth preferences of the above species and their proportions at sites they co-occur in the studied lake. Depth of occurrence and quantitative share of macrophytes were studied in a 1-m depth gradient along 12 transects in July 2016 and 2017. Physico-chemical water parameters evidenced meso-eutrophic status of the lake. Out of 23 taxa of aquatic macrophytes identified in Lake Kuźnickie high frequencies and abundances were recorded only for L. barbatus, N. obtusa, C. demersum and M. spicatum, thus being predominants in the lake’s vegetation. L. barbatus co-occurred with higher trophy indicators not only in the same lake but also at the same sites. Ceratophyllum demersum, M. spicatum and N. obtusa were negatively correlated with depth. Lychnothamnus barbatus preferred deeper sites than other predominants, as evidenced by ANOVA. Whilst M. spicatum showed no statistically significant relationship with L. barbatus, C. demersum and N. obtusa were negatively correlated with this endangered charophyte. We postulate that due to shade tolerance and light use efficiency L. barbatus is a better competitor at deeper sites compared to angiosperms and N. obtusa.

Temperature and depth preferences of adult sea trout Salmo trutta during the marine migration phase

Temperature and depth preferences of adult sea trout Salmo trutta during the marine migration phase

Layered Scene Models from Single Hazy Images

This paper describes the construction of a layered scene model, based on a single hazy image that has sufficient depth variation. A depth map and radiance image are estimated by standard dehazing methods. The radiance image is then segmented into a small number of clusters, and a corresponding scene plane is estimated for each. This provides the basic structure of a layered scene model, without the need for multiple views, or image correspondences. We show that problems of gap filling and depth blending can be addressed systematically, with respect to the layered depth structure. The final models, which resemble cardboard 'pop-ups', are visually convincing. An implementation is described, and subjective depth preferences are tested in a psychophysical experiment.

Bowhead and Beluga Whale Distributions, Sighting Rates, and Habitat Associations in the Western Beaufort Sea in Summer and Fall 2009–16, with Comparison to 1982–91

We analyzed data from line-transect aerial surveys for marine mammals conducted in the western Beaufort Sea (shore to 72˚ N, 140˚–157˚ W) from July to October of 2009–16 to investigate the distribution, behaviors, sighting rates, and habitat use preferences of bowhead and beluga whales. The habitat use data allowed for direct comparison with data collected in the same area from 1982 to 1991. Both species are ice-adapted, migrating through leads in sea ice in spring, and are seasonal inhabitants of the western Beaufort Sea during summer and fall. From 2009 to 2016, bowheads were seen in all survey months, with the highest overall sighting rate (whales per km) in August. Bowhead sighting rates were highest in the whales’ preferred habitats: outer shelf habitat (51–200 m depth) in July and inner shelf-shallow habitat (≤ 20 m depth) in August, September, and October. Beluga whales were also seen in all survey months, with highest overall sighting rate in July. Beluga whales were overwhelmingly associated with continental slope habitat (201–2000 m depth) in all months. Bowhead distribution and depth preferences in summer months of 2009–16 differed from those observed in 1982–91, when bowheads were not seen during limited survey effort in July and preferred outer continental shelf habitat in August. These differences indicate that bowhead whale preference for shallow shelf habitat now occurs earlier in summer than it used to. Beluga distribution and depth preference remained similar between 1982–91 and 2009–16, with strong preference for continental slope during both periods. Differences in sea ice cover habitat association for both species are likely due more to the relative lack of sea ice in recent years compared to the earlier period than to shifts in habitat preference. Habitat partitioning between bowhead and beluga whales in the western Beaufort Sea remained evident except in July, when both species used continental slope habitat. In July – October 2009–16, the distribution, sighting rates, and behavior of both bowheads and belugas in the western Beaufort showed considerable interannual variation, which underscores the importance of annual sampling to accurate records of the complex western Beaufort Sea ecosystem.

Distribution of Engraulis encrasicolus eggs and larvae in relation to coastal oceanographic conditions (the South-western Adriatic Sea case study)

Identification of potential spawning and nursery areas of European anchovy (Engraulis encrasicolus) represents an essential step in the management of a resource which is of fundamental importance both for fishery and pelagic trophic web. Egg and larvae occurrence from ichthyoplankton surveys (2012 - 2015) in the South Western Adriatic Sea were examined to understand the mechanisms that control their distribution. Egg and larvae densities varied through the years with the highest values recorded in 2012 and the lowest in 2014. Positive correlations between eggs and larvae with zooplankton were observed. When quotient analysis was used to find relations with environmental and biological variables, the results pointed out an egg preference for depth between 91 and 120 m and an avoidance between 11 and 30 m. Moreover, egg avoidances for high values of chlorophyll-a (&gt; 0.52 mg m-3) and low values of zooplankton biomass (&lt; 199 mg m-2) were highlighted. On the other hand, different ranges of habitat preferences and avoidances were observed for larvae: preference range of depth between 11 and 60 m and avoidance &gt; 151 m; preference for high zooplankton biomass (&gt; 1000 mg m-2) and avoidance for low biomass (&lt; 299 mg m-2). These correlations and the quotient values suggest that egg and larvae distribution in the South-Western Adriatic Sea is mainly driven by food availability and depth.

Depth and Seasonal Effects on the Settlement Density of Two Mussel Species (Perna Perna and Mytilus Galloprovincialis) in Offshore, Agadir (Morocco)

A study of spat settlement of two mussel species was carried out in Agadir area on the North Atlantic Coast, between Jan 2002 and Jan 2003. The preferred depth of settlement and settlement period of both species were monitored on collectors suspended in offshore at three different depths (1, 5, 10 m). For Perna perna, the effect of season on settlement was consistent, with relatively higher settlement both in spring (286-462 spats.m-1 ) and in summer (406-594 spats.m-1 ). Similar abundances of settlers were found at 1 m and 5 m depth whatever the season, which suggests a homogeneous distribution of settlers of P. perna in the first 5 m of the sea water. For Mytilus galloprovincialis, the settlement was less patchy in time in regard to P. perna (ρ&lt;0.05). Thus, the settlement was continuous from spring to autumn until 10 m depth. The filamentous structures (laces in polypropylene) used in this study are often designed to enhance the amount of settlers, however, the settlement densities of both species were very low. Consequently, the results suggest that larval supply has been the limiting factor in the settlement success, but not the lack of suitable substrates. Moreover, the study area has poor spat falls and seems to be not suitable for collection of mussel spat.