Aggregations of the jellyfish Aurelia labiata:abundance, distribution, association with age-0 walleye pollock, and behaviors promoting aggregation in Prince William Sound, Alaska, USA

Abstract

Aurelia labiata medusae occurred in aggregations with hundreds to millions of jellyfish. The aggregations were widely distributed in inlets of Prince William Sound (PWS), Alaska. Aerial surveys of PWS in May to August in 1995August in , 1996August in , and 1997 showed marked interannual variation in the numbers of aggregations observed, from a minimum of 38 in 1997 to a maximum of 557 in 1996 Acoustic surveys showed that the aggregations extended from near-surface (0 to 5 m) to 15 m depth. Schools of young-of the-year walleye pollock Theragra chalcogramma were associated with A. labiata, both within and below the aggregations. All seine catches that contained juvenile pollock also contained jellyfish. Medusa swimming was analyzed from underwater videotapes in order to elucidate how aggregations might be formed and maintained. Medusae did not swim in circles in the aggregat i o n~. Medusae all swam vertically in the same direction, either up or down, in crowded parts of the aggregations, suggesting that they had become concentrated in flow features, like convergences, in the water column. Reduced swimming, due pnmarily to frequent collisions among medusae in the aggregation~, also may have caused the medusae to become concentrated. The potential advantages of aggregation include increased fertilization success, retention near shore where planula settlement sites and zooplankton prey may be more abundant, retention in convergences where zooplankton are concentrated, and reduced predation.

Prey selection and dietary overlap among zooplanktivorous jellyfish and juvenile fishes in Prince William Sound, Alaska

“…Similarly, in July 1996, A. labiata and herring were more abundant and pollock less abundant than in 1999, and 20 to 25% of seine sets that targeted on fish schools also contained jellyfish (Purcell et al 2000). Additionally, juvenile walleye pollock were observed by underwater video to be associated with aggregations of A. labiata in 1996 and with individual C. capillata in 1999 (Purcell et al 2000). Clearly, the jellyfish and fish species included in this study overlap spatially and temporally and utilize the same foods in PWS.…”

Developing a simple, rapid method for identifying and monitoring jellyfish aggregations from the air

“…For this we required a technique applicable over broad areas spanning thousands of square kilometres yet providing detailed information on the aggregations themselves. Although previous studies have touched upon these issues (Purcell et al 2000, Graham et al 2003a, few have focused in detail on the methods employed per se. Our study aimed at highlighting the advantages and disadvantages of this potentially important technique for the study of gelatinous zooplankton.…”

“…Indeed, recent studies have drawn attention to their potential for the study of scyphozoan jellyfish that form large aggregations near the surface (Purcell et al 2000, Graham et al 2003a. Logically, this is limited to relatively large and visibly detectable (i.e.…”

“…non-transparent) species that aggregate, at least some of the time, in the top few metres of the water column. For example, Purcell et al (2000) used visual observations from aircraft to estimate the number of Aurelia labiata aggregations found throughout Prince William Sound, Alaska. By conducting surveys over 3 consecutive years, these authors were able to detect marked interannual variations in the number of aggregations observed, from a minimum of 38 in 1997 to a maximum of 557 in 1996.…”

Identifying potentially harmful jellyfish blooms using shoreline surveys

“…In situ monitoring is a powerful tool which gives a snapshot of what is in the water column at a particular time; however, it does not give a geographical context for the threats that are present outside of the immediate area at that time. Aerial surveys (Purcell et al 2000, Graham et al 2003, Houghton et al 2006, Nickell et al 2010) and remote sensing (Nickell et al 2010) have been tested successfully for large-scale jellyfish surveys but can be expensive and weather dependent. Shoreline surveys are not the complete solution but they are easily repeatable, relatively inexpensive and can provide a broad backdrop to in situ monitoring techniques.…”