title for limpet section of the Odyssey
   
  Harvesting by humans
 

photograph of owl limpet Lottia gigantea courtesy Jackie Soanes, Bodega Marine Laboratory, CaliforniaAlthough no commercial fisheries has existed for limpets on the west coast, there has been a long tradition of recreational consumption. Nowadays this is relevant only to the harvesting of owl limpets Lottia gigantea in northern Baja California. Photograph courtesy Jackie Soanes, Bodega Marine Laboratory, California.

NOTE  most or all west-coast First-Nations Peoples have had a tradition of eating limpets.  They are easy to harvest and yield a small nugget of flesh that, while chewy, is tasty and protein-rich

  black dot
Research study 1
 

map of study sites showing human exploitation of owl limpts Lottia gigantea in southern California and Baja MexicoHuman gatherers have traditionally used owl limpets Lottia gigantea as a food source.  The species is the largest in North America, reaching over 50mm shell length, and is reputed to be quite tasty.  Over-harvesting to the point of extinction is unlikely in the U.S., where at least some populations enjoy protective management in preserves (e.g., Channel Islands, California), but are populations at greater risk in less carefully managed areas, such as along the Baja California coast?  To assess this, researchers compare sizes of Lottia in the field, and in “catches” (=current harvests) and in some archeological middens along a gradient of intertidal exploitation. Mean shell lengths range from 50mm in 0-rated (protected from harvest) sites to 32mm in 3-4-rated (maximally exploited) sites.  Shell sizes in catches from these exploited sites range around 28mm.  Thus, sizes decrease along an exploitation gradient.  Moreover, to the extent that the limited catch data presented by the authors allows, the difference between mean size in intertidal populations and catches become smaller as level of exploitation increases.  Examination of middens reveals that older sites have larger-sized shells than do recent sites.  Thus, collecting of L. gigantea has occurred and is occurring unless protected by inaccessibility (topography or distance) or other restrictions.  Although certain characteristics of the Lottia populations that would tend to mitigate risk of extinction from over-collection are absent, such as: 1) ability of some limipets to be hidden from collectors (in cracks or burial), 2) presence of subtidal breeding populations, and 3) migration of adults from protected sites, two other attributes make extinction unlikely.  The first is that owl limpets mature at a small size, 25mm – a size actually smaller than the smallest individuals observed in catches.  The other attribute is their pelagic larval phase, which means that continued recruitment can occur from populations in protected sites.  Pombo & Escofet 1996 Pac Sci 50: 393;  for a review of ecological and evolutionary effects of such size-selective harvesting in both marine and terrestrial habitats see Fenberg & Roy 2008 Mol Ecol 17: 209; for another account of historical decline in body size of 4 rocky-shore gastropods including Lottia gigantea see Roy et al. 2003 Ecol Letters 6: 205.

NOTE  exploitation is examined in 11 sites rated on a scale from 0 (no gathering because of protective management such as the Channel Islands, California, or restrictive topography such as Bufadora, Baja California) to 4 (year-long unrestricted gathering as at traditional picnic sites and/or where there is easy access by paved roads, such as several sites in Baja California)

 
Research study 2
 

Studies on owl limpets Lottia gigantea at 8 sites in southern California (Orange County: Newport Beach to Dana Point: 20km) show that maximum age in open-rock habitats is is 8-9yrs at a shell length of about 65mm.  This size is well below the maximum for this species (>100mm).  The large size and conspicuousness of L. gigantea make it susceptible to collection, either by shell collectors or by fisher-folk for bait or food, a practice that the authors note has been going on in the area for over 13,000yr.   Worse still, owl limpets are protandrous, meaning that the larger more highly susceptible individuals are females.  The authors’ data indicate that shell lengths throughout the study area are negatively correlated with number of human visitors, confirming that the bigger individuals are being removed.  Interestingly, approximately 60% of the study area is within longstanding Marine Protected Areas (from 1994: California Marine Life Refuges and Ecological Reserves) from which collecting of invertebrates is prohibited.  So, either the restrictions are not working, or the “rebound” potential of the Lottia populations is much slower than might be anticipated.  Kido & Murray 2003 Mar Ecol Progr Ser 257: 111.

NOTE  the authors remark that black oystercatchers Haematopus bachmani, known predators of owl limpets, have been absent from the area for 10yr

 
Research study 3
 

map showing collection sites for limpets Lottia gigantea used for genetic studiesAn interesting follow-up to Research Study 1 above asks the question as to what genetic change may have been induced in owl limpets Lottia gigantea through centuries of size-selective harvesting.  In general, exploited populations of owl limpets are known to have significantly lower biomass and reproductive output in comparison with protected populations. The map shows the collection sites for the present study, representing 3 protected and 5 unprotected populations.  The researchers, based at The Natural History Museum, London and at various U.S. institutions, report no significant genetic structure or differences in genetic diversity, suggesting that all tested individuals are part of a single interbreeding population and that there are no geographic barriers to dispersal.  This suggests that populations currently exploited should, if left alone, bounce back to their pre-harvesting state with respect to size, demography, reproductive output, and ecological interactions. Fenberg et al. 2010 Mar Ecol 31: 574.

NOTE  genetic structure is assessed using 6 microsatellite markers.  Obviously, the best way to determine effects of size-selective harvesting on genetic make-up is to analyse samples historically, as has been done with some fish and other commercially exploited species (with varying success).  The next best is to compare samples from exploited sites with protected marine-reserve sites, as is done here

NOTE  these represent limited-access beaches within a national park area, air-force base, and golf course.  The 5 “unprotected” sites are considered to have varying degrees of harvest

 
Research study 4
 

map showing study sites for owl-limpet survey on San Miguel Island, Californiagraph showing change in harvest length of owl limpets Lottia gigantea over 10,000 years in San Miguel Island, CaliforniaAn interesting study by a research consortium mostly from Oregon and California museums and universities investigates historical predation by humans on owl limpets Lottia gigantea on San Miguel Island, California (see map).  The group uses 1718 shells from 19 archaeological middens to document changes in shell size over time dating back 10,000yr ago (i.e., through most of the Holocene epoch).  Results show size reductions beginning more than 8000yr ago (mean shell size of about 80mm) to 200yr ago (shell size 35mm; see graph).  No clear correlations exist with estimated seawater temperatures or marine productivity from the time, suggesting that human gatherers are responsible.  The study adds to our appreciation of human impacts on nearshore fisheries during historical times - impacts that occurred at a much earlier time than previously thought. Erlandson et al. 2011 J Archaeological Sci 38: 1127; see also Sagarin et al. 2007 Mar Biol 150: 399 for more on long-term harvesting of L. gigantea on the California coast and islands.

NOTE  estimates of ages obtained from radiocarbon 14C-dating of midden components, the oldest of which is estimated as 10,000 years before present (ybp) and the youngest at 200ybp

NOTE  estimates obtained from sediment analysis from the Santa Barbara Basin

 
  RETURN TO TOP