Almost two decades of non-invasive comparisons of sessile invertebrate abundances on vertical subtidal photographic transects in a sanctuary at the edge of Sea of Cortez and the Pacific Ocean, Cabo San Lucas, Baja Sur, Mexico

Background/Question/Methods

Non-invasive field photographic sampling minimizes interference during repeated long-term sampling. Marine mollusca have provided long-term comparisons, at sites including central Sea of Cortez on the Baja Peninsula monitored by Bertsch, where species diversity increased but population densities decreased throughout his sampling, 1992 to 2001. Additional latitudinal comparisons in Sea of Cortez detected no latitudinal gradient, but western North American shores show very broad latitudinal differences among diverse intertidal invertebrates. Long-term, detailed comparisons of subtidal plots are very rare, but can monitor abundances or sizes of animals and plants hypothetically more stable, fully submerged. This present site is in an underwater sanctuary, long protected by the Mexican Navy and dive operators, where Sea of Cortez meets the Pacific at the southern tip of Baja, California. I established photographic transects in 1997, of ~30 cm x 60 cm contiguous quadrats, arranged vertically to a depth of 13 m. Tidal range is <1.6 m. Within each subtidal zone here, the central hypothesis was that some sessile individuals would tend to persist and grow, while other taxa would disappear or increase in abundance. The present study compared these subtidal photographic quadrats in mid April (cool season) of 1998, 2008, and 2014. 

Results/Conclusions

Water transparency (secchi depth) was < 12 m during the sampling, with mid April bottom temperatures of 20˚C. Subjective comparisons from these April photo quadrats and each of four seasons, throughout these decades since 1997, suggested no major seasonal fluctuations in these sessile mollusca and other major sessile taxa, although water temperatures reach 26˚C in late summer. These vertical rock surfaces were largely shaded, with little open space, erect algae, or coral, except a shallow subtidal zone of colonial cup coral (a species without symbiotic algae,) Tubastraea coccinea. At least two major subtidal depth zones were evident. Deeper corals included Pocillopora elegans, which suggested partial bleaching, then some recovery and growth of individual colonies. Photographic observations also tabulated abundance of larger mollusca, including numerous rock scallops (Hinnites giganteus), other mollusks, and other major sessile invertebrates. Although rock scallop shells often were encrusted with variable sponges, hydroids, barnacles, etc., sea fans were aquatic landmarks for large individual (edible) oysters growing detectably from the previous decade. Small oysters frequently appeared eaten by dense fishes. Some broken coral appeared on sandy shelves below these rocks, evidently from frequent divers, yet diverse sessile invertebrates appeared quite stable over these decades, with individuals growing successfully.