River otters are notoriously elusive, largely nocturnal, and difficult to observe directly. But they congregate at specific sites known as latrines, which they use primarily for scent-marking through defecation, urination, and anal gland secretions, though they also eat, play, socialize, and rest there. These latrines are persistent landscape features that can remain active for decades, littered with scat and identifiable by their fishy smell, tracks, and signs of habitat manipulation. That makes them ideal for non-invasive monitoring. The research team surveyed three transects along the Rhode River, a low-salinity subestuary on a 1,072-hectare SERC property with 22 kilometers of shoreline, and identified 36 areas of otter activity between October 2020 and August 2021. Of those, 25 qualified as active latrines under strict criteria requiring at least two scats deposited over multiple visits and concentrated in a focal area. Eighteen latrines ultimately had measurable habitat characteristics and were included in statistical analysis.

The habitat findings were striking in their consistency with prior models from inland riverine systems. Active latrines were significantly more often characterized by permanent beaches, an open water interface, drop-offs, and undercut banks than randomized control points. Focal point clearings averaged about 19 square meters, roughly 4 meters long and 4.5 meters deep, and were significantly more likely to have soil-based sediment and woody debris. Overstory and understory vegetation heights were significantly lower at latrines than at control sites, suggesting otters concentrate their marking activity under low-hanging branches, possibly for protection from predators or to shield scat from degradation and extend the longevity of scent signals. Slope incline and burrows were seldom present. The researchers note that while marshes comprise much of the SERC shoreline, otters instead chose areas with woody debris and open water access, characteristics broadly associated with latrines across riverine, lacustrine, and coastal systems.

Perhaps the most unexpected habitat finding was the otters’ ready adoption of manmade structures. Five latrines were found on floating and fixed docks, elevated boardwalks, and a shoreline staircase. Dock and boardwalk latrines showed some of the highest otter activity in the study area and were used for scat collection, despite boat traffic and proximity to actively used walking trails. The researchers suggest that developed shorelines may be a significant component of river otter habitat in increasingly urban landscapes, a finding with implications as otter populations expand into degraded and urbanized estuaries across North America, including highly urbanized areas of the Chesapeake Bay where otters have been spotted in Baltimore’s Inner Harbor and even on the National Mall in Washington, D.C.

To examine diet, the team collected 90 scats and deployed a dual approach: microscopic examination of prey remains and DNA metabarcoding of dietary DNA. For the genetic analysis, they amplified a roughly 133-base-pair fragment of the mitochondrial cytochrome oxidase I gene for prey identification and a roughly 130-base-pair fragment of the V9 region of the 18S ribosomal gene for parasite detection. The COI gene was chosen because it has a curated local database from the Chesapeake Bay Barcode Initiative with high taxonomic coverage, providing the highest resolution for species-level assignments. Sequencing generated nearly 9 million raw reads for COI, which after filtering, merging, chimera removal, and exclusion of otter sequences and negative controls, yielded 27 potential primary prey taxa detected 333 times across all scats. On average, metabarcoding detected 3.7 prey items per scat, with 57 percent identified as finfish and 36.6 percent as crustaceans.

The diet results confirm that Chesapeake Bay otters are opportunistic generalists consuming both aquatic and terrestrial fauna. The most abundant crustacean by sequence number, and the prey item found in the greatest number of scats, was the American blue crab, a commercially important and widespread species in the Bay. Finfish prey included species in the Cyprinodontidae and Clupeidae as most frequently consumed, along with anadromous species such as striped bass, gizzard shad, and American eel. Otters also opportunistically consumed freshwater devil crayfish, amphibians, and ducks. Two non-native species already established in Maryland waters, common carp and white river crayfish, appeared in scats as well. Microscopic examination of 89 scats identified 199 individual prey items across 18 taxa, with ray-finned fish remains occurring in 92.1 percent of scats and crustaceans in 52.8 percent.

The comparison between methods revealed the technical power of metabarcoding. Of 20 orders detected, 45 percent were found with both methods, 35 percent only with metabarcoding, and 20 percent only with microscopy. Soft-bodied organisms such as polychaete worms, finely scaled fish like American eel, and scaleless fish including catfish and gobies were identified only genetically, since they leave no recognizable scales behind. Metabarcoding also detected pickerel, silversides, herring, frogs, and crabs more frequently than microscopy, while killifish and flatfish were identified more often microscopically. The researchers note that prior studies relying solely on microscopic methods may have underestimated the variability of summer diets, when otter diet diversity tends to peak. However, microscopy retains advantages: it can provide information on prey size and volume consumed, which genetics cannot, and it allowed the researchers to distinguish true prey items, such as cicadas likely consumed during the summer 2021 Brood X emergence, from decomposers that colonized scat after deposition. The authors conclude that a combined morphological and genetic approach is required for a holistic assessment of prey consumed.

Beyond prey, the 18S metabarcoding opened a window into the parasite community within the food web. The analysis yielded 591,412 sequences across 138 amplicon sequence variants from 10 parasitic taxa in six classes. Trematodes, parasitic flatworms requiring multiple hosts throughout their life cycles, were the most frequently detected and dominated relative abundance at 99.5 percent of parasite sequences, with 81.3 percent belonging to the order Plagiorchiida. Diplostomid trematodes typically use fish as second intermediate hosts before reaching birds or mammals as definitive hosts, while Plagiorchiid trematodes typically use arthropods as second intermediate hosts and fish as final hosts. The team also recovered monogeneans, including Gyrodactylidea, which commonly infect fish gills, and multiple myxozoan taxa, many of which parasitize fish. Most parasites identified do not infect mammals, meaning they most likely infected the otters’ prey and were consumed along with it, a phenomenon ecologists call concomitant predation.

Some parasites, however, likely infected the otters themselves. Nematodes in the order Strongylida, primarily known to infect terrestrial vertebrates and marine mammals, were detected, a finding supported by observations of large red nematodes visible in fresh scat. Multiple apicomplexans were also recovered, including parasites in the Sarcocystidae, which infect a wide range of mammalian species. As river otters increasingly reside in urban areas, they face greater exposure to zoonotic parasites such as Cryptosporidium and Giardia, making non-invasive parasite surveillance through scat a potentially valuable public health tool. The researchers caution that metabarcoding cannot determine parasite life stages and that the short amplified fragment limits taxonomic resolution, suggesting future studies combine genetics with traditional fecal flotation or sedimentation methods to identify parasite eggs.

This study marks a critical first step in expanding ecological research on river otters across the increasingly urbanized Chesapeake Bay watershed. The latrine characteristics identified here can be used to locate latrines from which non-invasive scat studies can examine otter ecology and health across land- and seascapes. Because otters are generalist predators, examining their diet across seasons and years would reveal how their food consumption shifts with seasonal migrations of prey, particularly anadromous species like striped bass and American eel. The researchers expect that teleost fish may become more important prey during fall and winter months, consistent with prior studies, and call for future work across additional subestuaries to provide broader regional representation of river otter ecology. As apex aquatic predators whose diet reflects the diversity of available prey and parasites within aquatic habitats, river otters may prove to be sentinels of ecosystem condition in one of America’s most studied and most pressured estuaries.

Subject of Research: Ecology

Article Title: North American river otters consume diverse prey and parasites in a subestuary of the Chesapeake Bay

Article References: Wise, C. M., Serfass, T. L., Palmer, E. M., Yisrael, L., Burton-Reeder, K., & Pagenkopp Lohan, K. M. (2025). North American river otters consume diverse prey and parasites in a subestuary of the Chesapeake Bay. Frontiers in Mammal Science, 4, Article 1620318. https://doi.org/10.3389/fmamm.2025.1620318

Image Credits: AI Generated

DOI: 10.3389/fmamm.2025.1620318

Keywords: river otter, Chesapeake Bay, metabarcoding, parasites, diet, latrines, apex predator, DNA barcoding, North, American, river, otters

The work matters because maritime spatial planning, or MSP, is the European Union’s flagship instrument for organizing human uses of the sea. The EU’s Blue Economy directly employed nearly 3.59 million people in 2021 and generated a turnover of around 623.6 billion euros, and marine renewable energy, mainly offshore wind, saw gross profits jump 45 percent compared with 2020. But the same directive that requires Member States to establish national MSPs also demands an ecosystem-based approach, consistent with environmental legislation such as the Marine Strategy Framework Directive. To plan sustainably, planners need to understand the combined effects of existing and future activities on the environment, and that is precisely what Cumulative Effects Assessments, or CEAs, are meant to deliver. Despite numerous national and international initiatives, their application in MSP still needs further development.

The core problem is deceptively simple. Most existing CEA tools describe the relationship between an activity and a pressure in binary terms: an activity either causes a pressure or it does not. A scuba diver and a bottom trawler both cause abrasion, so in a normalized calculation their maximum contributions count equally, even though the trawler abrades square kilometers of seabed for every hour of fishing while the diver disturbs a few square meters. The researchers, led by Alexis Esquerré of Cerema with colleagues from the French Biodiversity Office and ILVO in Belgium, addressed this by attributing a magnitude value to each activity-pressure relationship, approximating the proportion of the environment affected by the activity. Sea bottom physical pressures were assigned magnitudes based on the area potentially affected by one unit of activity intensity, while modifications of seawater properties were based on the volume affected.

The tool they upgraded, the RCE Carpediem tool, is an open-source program written in the R language, first published in 2020 by the French Biodiversity Office. It uses relative intensities of human activities to estimate the intensity of the pressures they generate, then calculates a Risk of Cumulative Effect based on the surface area and sensitivities of benthic habitats to various pressures, delivering results on a grid covering the study area. The new version adds a third matrix listing the magnitude of each activity for each pressure, alongside the existing activity-pressure matrix and the sensitivity matrix of ecological components. The final weighted relationship between an activity and a pressure is defined as the product of a binary variable indicating whether the activity causes the pressure, and ten raised to the power of the corresponding magnitude, so a relationship with a magnitude of three carries a weight of one thousand.

One subtlety required careful handling. After summing the weighted contributions of activities, the tool applies a log-normalization that rescales each pressure between zero and one across the study area. That step is essential for comparing pressures described in different units, but it would also erase the magnitude correction: a pressure caused by a single activity of high magnitude and a pressure caused by a single activity of low magnitude would end up with the same range. To solve this distortion, the researchers readjusted each normalized pressure using the highest weighted relationship among all activities that can generate it, a precautionary choice consistent with the risk-assessment framework. The result is a calculation that preserves the relative scales at which different pressures are capable of affecting the environment.

The team also added something genuinely new: a module of activity unsuitability. For each grid cell, the module compares all the pressures generated by each activity, weighted by their magnitudes, against the total sensitivities of the habitats in that cell. An activity is more unsuitable wherever ecosystem components are highly sensitive to whichever pressures the activity emits at high magnitude. Crucially, unsuitability does not depend on how intensely the activity is currently carried out, which makes it a planning reference rather than a snapshot of present conditions. A companion activity risk indicator then multiplies unsuitability by actual activity intensity, flagging cells where an unsuitable activity is nonetheless being carried out intensely.

Two case studies tested the approach at contrasting scales. The first, at local scale, covered the Bay of Seine off the Seine river estuary on the French side of the English Channel, an area packed with harbor traffic from Le Havre, fishing, one operational offshore wind farm, one under construction, and one designated exploration zone. The second covered the Greater North Sea restricted to the maritime spaces of EU Member States, from France, Belgium and the Netherlands to Germany, Denmark and Sweden, including the Danish Straits, Kattegat and Skagerrak. The local study used a grid of one minute of degree per cell with national French datasets; the sea-basin study used larger three-minute cells with harmonized data from the European Marine Observation and Data Network, EMODnet. The grids align perfectly: each sea-basin cell contains exactly nine local cells.

The comparison of methods in the Bay of Seine produced a result that should reassure skeptical planners. Weighting pressures by their magnitude refined the calculation without altering the overall distribution of results: the decile boundaries of the risk series differed on average by only about 0.03 between the two methods, and the same broad areas of high and low risk emerged. The southern part of the bay, closest to the estuary where bottom trawling and traffic from Le Havre concentrate, remained a consistent hotspot. But the differences were telling. The binary method flagged two permanently present underwater cables in the north of the study area as a major source of risk, which the weighted method did not, since cables affect only a few square meters around them. Conversely, the weighted method revealed higher risk around the operational offshore wind farm in the east, an area the binary method underestimated because other activities are restricted inside wind farm boundaries, leaving the farm as the sole generator of certain pressures there.

At sea-basin scale, the assessment identified large areas of high risk off the northern coasts of Normandy and the Bay of Somme, in patches of the Danish Straits, east of the Doggerbank sand bank, and in the Skagerrak, while offshore areas near Denmark and off the western Netherlands showed lower risk. Importantly, the spatial distribution of risk differed from the distribution of total activity intensity, confirming that higher concentrations of activities do not necessarily produce higher calculated risk, and that the tool maps risk rather than merely activity. The Bay of Seine extract from the sea-basin results was broadly similar to the local results but less precise, with noise and artifacts attributed to the coarser habitat map. The researchers conclude that the question an assessment is meant to answer should drive the choice of scale: national planning benefits from local-scale detail, while international coordination needs a basin-wide reference.

The unsuitability results may prove the most immediately useful for planners. In the Bay of Seine, pelagic trawling was generally far more unsuitable for benthic habitats than maritime traffic, yet both activities showed comparable unsuitability values above one thousand near the Seine estuary, and traffic was considerably more unsuitable there than across most of the bay. In the Greater North Sea, pelagic trawling unsuitability was higher in offshore areas and the Danish Straits than in the Channel. The authors caution that data accuracy, especially habitat sensitivities, is crucial: habitats with few known sensitivities, such as patches classified at low EUNIS detail in the Skagerrak, can depress calculated unsuitability and mask real conflicts. Remaining challenges include expert-judgment sensitivity values, the poor integration of food-web pressures and temporal dynamics, and the fact that the tool cannot yet credit activities with positive effects, such as wind farms acting as artificial reefs. Still, by weighting pressures by their real spatial reach and revealing where sensitive habitats collide with high-magnitude activities, the upgraded framework gives ecosystem-based maritime spatial planning a sharper instrument than it has ever had.

Subject of Research: Implementation of cumulative effect assessments in maritime spatial planning: insights from two North Sea case studies

Article Title: Implementation of cumulative effect assessments in maritime spatial planning: insights from two North Sea case studies

Article References: Esquerré, A., Quemmerais-Amice, F., Gimard, A., Van Hoey, G., Quentric, A., & Laroussinie, O. (2026). Implementation of cumulative effect assessments in maritime spatial planning: insights from two North Sea case studies. Frontiers in Ocean Sustainability, 4, Article 1800336. https://doi.org/10.3389/focsu.2026.1800336

Image Credits: AI Generated

DOI: 10.3389/focsu.2026.1800336

Keywords: Implementation, cumulative, effect, assessments, maritime, spatial, planning, insights, North, case, studies, scientific research