Mass mortality of sea urchins is not uncommon, as witness several occurrences in California.  The latest report of these concerns red sea-urchins Strongylocentrotus franciscanus in 2 regions near Santa Cruz (Año Nuevo Island and Point Santa Cruz).  At the time of publication the disease agent is unknown but manifests itself in spine loss and test plates layered with a thin greenish surface layer and red friable middle layer, overlying a normal white inner layer.  The authors characterise the mortalities as localised.  Pearse et al. 1977 Fish Bull 75: 645.

Diseased Strongylocentrotus franciscanus.
The cause is not known, but possibly fungal 0.6X

This is followed by a large increase in canopy cover of brown kelps over the following year. The area of major increase is shown in brown colour on the maps below. Initially, there are 4 species of kelps but, within a year, the competitively dominant giant kelp Macrocystis pyrifera, a favoured food of red sea urchins, becomes a virtual monoculture. Pearse & Hines 1979 Mar Biol 51: 83.

NOTE  these are M. pyrifera, Laminaria dentigera, Pterygophora californica and, to a lesser extent, Nereocystis luetkeana

NOTE the large blades (fronds) of the kelp have gas-filled bladders that float them at the surface.  The area of fronds floating in this way represents the canopy cover of the kelp. The photo shows a northern species of kelp Macrocystis integrifolia with the floats visible.

In the Point Loma, San Diego area of California storms and warm water associated with El Niño events during 1982-84 are thought to have been the cause of major losses in giant kelps Macrocystis pyrifera.  Intensive fisheries for red sea-urchins Strongylocentrotus franciscanus in the area during the late 1970s and into the early 1980s, however, may have contributed to the eventual recovery of the kelp-beds.  Tegner & Dayton 1991 Mar Ecol Progr Ser 77: 49.

NOTE the graph shows temperature records from 3 sites in the kelp forest, the first 2 peaks representing the causative El Niño events

An embayment in Barkley Sound full of kelp. Nearshore
types are Laminaria spp. and Pterygophora sp, extending to
Macrocystis integrifolia and Nereocystis luetkeana offshore

Studies of disease in purple sea-urchins Strongylocentrotus purpuratus at several locations in southern California and northern Baja California, Mexico disclose the presence of 2 distinct pathologies, rather than just a single one expected from previous studies.  Both pathologies are common in warmer southern waters, but are rare or absent in the cooler waters north of Point Conception (see graph).  Laboratory experiments show that elevated water temperature leads to larger lesions and significantly lower gonadal indices, suggesting that thermal stress may exacerbate the urchins’ susceptibility. Although a density dependence in disease prevalence is expected, this only occurs with the black-ring pathology, and then in negative relationship; that is, lower disease prevalence correlates with higher densities. Of the 2 diseases, black-ring appears to be most prevalent, with 2 of the Baja California populations exhibiting up to 15% infection in autumn (see graph).  In comparison, prevalence of red-spot disease is generally less than 5%, save for one site in California (South Laguna Beach) that reaches 15% prevalence also in autumn.  The authors note that if thermal stress does increase the urchins’ susceptibility to disease, then warming associated with climate change may have significant effects on the economies of kelp forests and sea-urchin fisheries.  Lester et al. 2007 Can J Fish Aquat Sci 64: 314.

NOTE  the 2 pathologies are “red-spot”, characterised by shortened spines and large areas of the test lacking spines, and “black-ring”, characterised by an outer black ring, and inner lighter area lacking spines and epithelium