The potential for hull‐mediated species transfers by obsolete ships on their final voyages

Abstract

Shipping has contributed strongly to biological invasions in coastal ecosystems, transferring species in ballast tanks and on exposed underwater surfaces (hulls). A long history exists that documents biota associated with ships’ hulls, including some recent analyses of modern ships, but relatively little is known about the associated risks of invasion. In general, the likelihood of invasion is expected to increase with increasing propagule supply, which suggests that high‐density transfers on hulls may pose a relatively high invasion risk. Obsolete vessels are expected to be at an extreme end of the spectrum for biofouling, since they sit at anchorage for long periods and are towed at relatively slow speeds when moved, but this remains largely unexplored. In this paper, we quantified the biofouling communities of two obsolete vessels, one stationary for one decade and the other for two decades, before and after their final transit from California to Texas. Pre‐departure biofouling surveys across both vessels detected 22 species of macroinvertebrates. The biomass was dominated by the introduced bryozoan Conopeum chesapeakensis, which occurred in 98% of samples and created a three‐dimensional structure (2–5 cm thick). Mobile species, inhabiting the vertical biofouling matrix, were more numerous than sessile ones. Interestingly, the non‐native Asian clam Corbula amurensis, not previously associated with hull fouling assemblages, was recorded in 9% of samples. During the 43‐day voyage, organisms encountered salinity variation that ranged between zero (Panama Canal) and at least 37 parts per thousand (Brownsville, Texas) and temperatures that varied between 9.9 °C and 31.6 °C. Upon arrival in Texas, we measured an expected decrease in biofouling extent across both vessels but also a surprising increase in species richness (57 species were recorded), with small compositional differences between ships that did not exist prior to departure. Several species were recorded alive upon arrival, including non‐natives that are not known to be established in Texas waters. The physiological tolerance and associated risk of colonization have not yet been evaluated for these organisms, or for the broader species pool associated with a standing fleet (n > 200 ships) that may undergo similar movements. Nonetheless, a compelling case exists for vector management based on organism flux alone, to reduce the risk of coastwise and inter‐oceanic invasions.

Interrupting a multi-species bioinvasion vector: The efficacy of in-water cleaning for removing biofouling on obsolete vessels

“…The SCAMP method of in-water scrubbing can be considered favorable to no pre-departure management effort, however, even though we recorded viable specimens from a majority of species on the ORION after scrubbing. Moreover, comparisons between this study and another of two similar vessels (Davidson et al, 2008), suggest that in-water cleaning is more effective at creating bare space and removing organisms than transit alone. Holm et al (2003) have demonstrated that several factors, including brush position, brush angle, rotation rate, bristle density and bristle stiffness, interact in a complex manner to determine the forces required to remove organisms from submerged surfaces.…”

“…The high levels of biofouling with little variation among hull locations found on the ORION represent common features of stationary vessels that receive little or no hull husbandry (Brock et al, 1999;DeFelice, 1999;Coutts, 2002;Minchin and Gollasch, 2003;Davidson et al, 2008). Such vessels tend to have extensive biofouling assemblages in both hull and non-hull areas that exceed niche area fouling of most operational ships.…”

“…It is clear that in-water cleaning reduces organism numbers. It is also likely that cleaning followed by movement (towing) would cause a further reduction, but this has not yet been estimated and another study has shown how species richness can increase during transit (Davidson et al, 2008). Furthermore, current post-voyage ship dismantling practice involves vessels being dragged out of water iteratively, over a period of weeks, as the vessel is taken apart.…”

“…These vessels are dwarfed in number by active commercial and military vessels around the world, but account for a far higher risk of species introductions on a per-ship basis. Factors such as long lay-up times, slow speeds and lengthy durations at recipient ports, combined with a lack of hull maintenance, ensure that very dense and extensive assemblages of biofouling organisms get relocated and provided with an opportunity to establish in new areas (DeFelice, 1999;Davidson et al, 2008).…”

Proceedings of the Smithsonian Marine Science Symposium

“…Alternatively, the service time of vessels may decrease as the capacity to accommodate more transits increases. This decrease could reduce the establishment probability of organisms attached to the hulls of arriving vessels, as residence time and likelihood of invasion are thought to be positively correlated (Davidson et al, 2008).…”