Marine animal forests as useful indicators of entanglement by marine litter

Abstract

Entanglement of marine fauna is one of the principal impacts of marine litter, with an incidence that can vary strongly according to regions, the type and the quantity of marine litter. On the seafloor, areas dominated by sessile suspension feeders, such as tropical coral reefs or deep-sea coral and sponge aggregations, have been termed “animal forests” and have a strong potential to monitor the temporal and spatial trends of entanglement by marine litter, especially fishing gears. Several characteristics of these organisms represent advantages while avoiding constraints and bias. Biological constraints and logistical aspects, including tools, are discussed to better define a strategy for supporting long-term evaluation of accumulation and entanglement of marine litter.

Introduction

Entanglement of marine fauna is one of the principal impacts of marine litter. The number of species known to have been affected by either entanglement or ingestion of plastic debris has doubled since 1997, from 267 to 557 species among all groups of wildlife (Laist, 1997; Kühn et al., 2015). Strong increases in records were listed for sea turtles (now 100% of the 7 species), marine mammals (now 66% of the 123 species) and seabirds (now 50% of 406 species). For entanglement only, the proportion of seabirds impacted ranges from 25% (in Kühn et al., 2015) to 36% (Ryan, 2018) and according to UNEP (2016), entanglement incidents in marine debris lead to wounds or death for a large number of other taxa, including 192 species of invertebrate and 89 species of fish. In a more recent review, entanglement was reported in 418 species from reef systems across eight taxa, also evaluating their major conservation implications (Carvalho-Souza et al., 2018).

A reduction in food intake is one of the most frequent consequences of entanglement, as well as, for mobile species, limitations in movements and thus escaping from predators (Kühn et al., 2015). Entanglement also leads to wounds susceptible to secondary infections and sometimes amputation after constriction (NOAA, 2014). Benthic organisms can also be caught in derelict traps or other litter items on the seafloor. For example, crabs, octopus, fishes and many small invertebrates are commonly captured in lost traps and nets, eventually dying because of starvation (June, 1990; Erzini et al., 2008; Good et al., 2010; Cho, 2011).

Although entanglement has been documented in many different types of debris, most records involved fishing gears, especially abandoned, lost, or otherwise discarded fishing gear (ALDFG), with an incidence that can vary strongly according to regions, the type and the quantity of marine litter.

Generally, the factors that may influence the probability of an organism being entangled in – or strangled by litter includes the size and structure of the debris, water turbidity, water depth but also behavioural traits (Kühn et al., 2015). For example, the lack of experience of juvenile or immature individuals can make them more vulnerable of being entangled in mesh nets. In young marine mammals, their “playful” and curious behavior has been suggested to increase the incidence of entanglement (Hanni and Pyle, 2000). In addition, ambient noise can hide or distort the echoes produced by ALDFG thus reducing the ability for cetaceans to detect nets by echolocation (Kühn et al., 2015). ALDFG can have an impact on the environment in many different ways, including the continuing catch of target species, the catching of non-target species or the entanglement of organisms, and the physical impact of gear on the benthic environment (Gregory, 2009), collectively termed “ghost fishing”. In some cases, ALDFG may represent almost 100% of total debris, especially in fishing grounds (Pham et al., 2014; Consoli et al., 2018) with monofilament fishing lines perhaps as the most dangerous kind of litter, as they represent a large part of entanglement records (Consoli et al., 2018). Many studies have described entanglement of epibenthic organisms in ALDFG, reaching up to 66% of the entangled or smothered benthic organisms, in particular for habitat-builders such as corals and sponges (Van den Beld et al., 2017; Melli et al., 2017).