This article develops methods for fitting spatial models to line transect data. These allow animal density to be related to topographical, environmental, habitat, and other spatial variables, helping wildlife managers to identify the factors that affect abundance. They also enable estimation of abundance for any subarea of interest within the surveyed region, and potentially yield estimates of abundance from sightings surveys for which the survey design could not be randomized, such as surveys conducted from platforms of opportunity. The methods are illustrated through analyses of data from a shipboard sightings survey of minke whales in the Antarctic.

Spatial models for line transect sampling

The habitat use of marine megafauna emerges from the complex interplay between access to patchy and variable food resources and several intrinsic biological factors, such as the interaction with conspecifics and offspring care, resulting in dynamic distribution patterns. Quantifying species' relationships with the underlying environment is further complicated by the scale-dependent nature of these processes. Multi-scale analyses that incorporate aspects of a species' biology and build on large datasets are therefore required to understand long-term distribution and inform appropriate management measures. In this study, we use monitoring data collected over two study periods (2003–2008 and 2012–2018) to assess the habitat use, trend in local occurrence, and change in distribution of sperm whales, Physeter macrocephalus, around the Balearic Islands (Spain), one of the few recognised breeding and feeding grounds for the ‘Endangered’ population in the Mediterranean Sea. Moreover, we investigate the differences in the habitat use of single animals and groups, to explore intra-specific niche partitioning in this highly social but behaviourally dimorphic species. Results suggest that overall the occurrence of sperm whales in the area has been increasing over time. Animals were found to associate with distinct bathymetric features, but the mechanisms generating these relationships, and the underlying oceanographic processes within this habitat, remained uncertain. Sperm whale distribution also underwent a significant shift between the two study periods, with an increased occurrence in the Mallorca channel and north of Menorca, which further points towards a dynamic use of the broader bathymetric range preferred around the archipelago. Finally, our analyses highlighted that single animals and groups used areas with different characteristics, with groups preferring deeper, warmer waters characterised by lower sea level anomaly, which resulted in some fine-scale spatial segregation. The results of this study shed light on the mechanisms underpinning the biogeography and complex social system of the species, and support the design of targeted conservation measures in this important breeding and feeding ground.

/pdf/multi-scale-analysis-reveals-changing-distribution-patterns-4fpvne4pjf.pdf

Multi-scale analysis reveals changing distribution patterns and the influence of social structure on the habitat use of an endangered marine predator, the sperm whale Physeter macrocephalus in the Western Mediterranean Sea

A comparative study on habitat models for adult bigeye tuna in the Indian Ocean based on gridded tuna longline fishery data

https://onlinelibrary.wiley.com/doi/pdf/10.1111/ecog.04979

Geographical differences in habitat relationships of cetaceans across an ocean basin

Background The Mediterranean subpopulation of fin whale Balaenoptera physalus (Linnaeus, 1758) has recently been listed as Vulnerable by the IUCN Red List of threatened species. The species is also listed as species in need of strict protection under the Habitat Directive and is one of the indicators for the assessment of Good Environmental Status under the MSFD. Reference values on population abundance and trends are needed in order to set the threshold values and to assess the conservation status of the population. Methods Yearly summer monitoring using ferries as platform of opportunity was performed since 2008 within the framework of the FLT Med Network. Data were collected along several fixed transects crossing the Western Mediterranean basin and the Adriatic and Ionian region. Species presence, expressed by density recorded along the sampled transects, was inspected for assessing interannual variability together with group size. Generalized Additive Models were used to describe density trends over a 11 years' period (2008-2018). A spatial multi-scale approach was used to highlight intra-basin differences in species presence and distribution during the years. Results Summer presence of fin whales in the western Mediterranean area showed a strong interannual variability, characterized by the alternance of rich and poor years. Small and large groups of fin whales were sighted only during rich years, confirming the favorable feeding condition influencing species presence. Trends highlighted by the GAM can be summarized as positive from 2008 to 2013, and slightly negative from 2014 to 2018. The sub-areas analysis showed a similar pattern, but with a more stable trend during the second period in the Pelagos Sanctuary sub-area, and a negative one in the other two sub-areas. Our findings further confirm the need for an integrated approach foreseeing both, large scale surveys and yearly monitoring at different spatial scales to correct and interpret the basin wide abundance estimates, and to correlate spatial and temporal trends with the ecological and anthropogenic drivers.

Trends in summer presence of fin whales in the Western Mediterranean Sea Region: new insights from a long-term monitoring program

Aim: Deep‐diving cetaceans are oceanic species exposed to multiple anthropogenic
pressures including high intensity underwater noise, and knowledge of their
distribution is crucial to manage their conservation. Due to intrinsic low densities,
wide distribution ranges and limited presence at the sea surface, these species are
rarely sighted. Pooling data from multiple visual surveys sharing a common linetransect
methodology can increase sightings but requires accounting for heterogeneity
in protocols and platforms.
Location: North Atlantic Ocean and Mediterranean Sea.
Time period: 1998 to 2015.
Major taxa: Ziphiidae; Physeteriidae; Kogiidae.
Methods: About 1,240,000 km of pooled effort provided 630 sightings of ziphiids,
836 of physeteriids and 106 of kogiids. For each taxon, we built a hierarchical
model to estimate the effective strip width depending on observation conditions
and survey types. We then modelled relative densities in a generalized additive
modelling framework. Geographical predictions were limited to interpolations
identified with a gap analysis of environmental space coverage.
Results: Deeper areas of the North Atlantic gyre were mostly environmental extrapolation
in the predictions, thereby highlighting gaps in sampling across the different
surveys. For the three species groups, the highest relative densities were
predicted along continental slopes, particularly in the western North Atlantic
Ocean where the Gulf Stream creates dynamic frontal zones and eddies.
Main conclusions: Pooling a large number of surveys provided the first basin‐wide
models of distribution for deep‐diving cetaceans, including several data‐deficient
taxa, across the North Atlantic Ocean and the Mediterranean Sea. These models can
help the conservation of elusive and poorly known marine megafauna.

Combining multiple visual surveys to model the habitat of deep-diving cetaceans at the basin scale

Marine mammals have been proposed as ecosystem sentinels due to their conspicuous nature, wide ranging distribution, and capacity to respond to changes in ecosystem structure and functioning. In southern European Atlantic waters, their response to climate variability has been little explored, partly because of the inherent difficulty of investigating higher trophic levels and long lifespan animals. Here, we analysed spatio-temporal patterns from 1994 to 2018 of one of the most abundant cetaceans in the area, the common dolphin (Delphinus delphis), in order to 1) explore changes in its abundance and distribution, and 2) identify the underlying drivers. For that, we estimated the density of the species and the centre of gravity of its distribution in the Bay of Biscay (BoB) and tested the effect of three sets of potential drivers (climate indices, oceanographic conditions, and prey biomasses) with a Vector Autoregressive Spatio Temporal (VAST) model that accounts for changes in sampling effort resulting from the combination of multiple datasets. Our results showed that the common dolphin significantly increased in abundance in the BoB during the study period. These changes were best explained by climate indices such as the North Atlantic Oscillation (NAO) and by prey species biomass. Oceanographic variables such as chlorophyll a concentration and temperature were less useful or not related. In addition, we found high variability in the geographic centre of gravity of the species within the study region, with shifts between the inner (southeast) and the outer (northwest) part of the BoB, although the majority of this variability could not be attributed to the drivers considered in the study. Overall, these findings indicate that considering temperature alone for projecting spatio-temporal patterns of highly mobile predators is insufficient in this region and suggest important influences from prey and climate indices that integrate multiple ecological influences. Further integration of existing observational datasets to understand the causes of past shifts will be important for making accurate projections into the future.

The Role of Climate, Oceanography, and Prey in Driving Decadal Spatio-Temporal Patterns of a Highly Mobile Top Predator

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Combining multiple visual surveys to model the habitat of deep-diving cetaceans at the basin scale

The Role of Climate, Oceanography, and Prey in Driving Decadal Spatio-Temporal Patterns of a Highly Mobile Top Predator