Recurrent jellyfish blooms are a consequence of global oscillations

Abstract

A perceived recent increase in global jellyfish abundance has been portrayed as a symptom of degraded oceans. This perception is based primarily on a few case studies and anecdotal evidence, but a formal analysis of global temporal trends in jellyfish populations has been missing. Here, we analyze all available long-term datasets on changes in jellyfish abundance across multiple coastal stations, using linear and logistic mixed models and effect-size analysis to show that there is no robust evidence for a global increase in jellyfish. Although there has been a small linear increase in jellyfish since the 1970s, this trend was unsubstantiated by effect-size analysis that showed no difference in the proportion of increasing vs. decreasing jellyfish populations over all time periods examined. Rather, the strongest nonrandom trend indicated jellyfish populations undergo larger, worldwide oscillations with an approximate 20-y periodicity, including a rising phase during the 1990s that contributed to the perception of a global increase in jellyfish abundance. Sustained monitoring is required over the next decade to elucidate with statistical confidence whether the weak increasing linear trend in jellyfish after 1970 is an actual shift in the baseline or part of an oscillation. Irrespective of the nature of increase, given the potential damage posed by jellyfish blooms to fisheries, tourism, and other human industries, our findings foretell recurrent phases of rise and fall in jellyfish populations that society should be prepared to face.

Fig. 1.

Distribution of jellyfish time-series. Time-series of observations on jellyfish abundance >10 y in length from 1874 to 2011. The diameter of the symbols is proportional to the duration of the dataset, colors indicate trends (linear regressions, P < 0.05): significant decrease (blue), significant increase (red), or no trend (gray) in jellyfish abundance over time for the duration of the study. Numbers identify the datasets described in Table S1. Most datasets were from the northern hemisphere (87%), in particular the Atlantic Ocean (17%) and the Mediterranean region (17%), and comprised medusae (89%).

Fig. 2.

Individual time-series of standardized jellyfish abundance from 1940 to 2011. Line colors show significant increase (red), decrease (blue), and no significant change (black) in jellyfish abundance over time (t test, P < 0.05). (Inset) The slopes of linear regressions for individual time series calculated over time periods of 1940–2011 and 1970–2011. Colored bars signify negative (blue) and positive (red) slopes; open bars show no significant deviation from 0 (t test, P < 0.05). Abbreviations of regions included in analysis and regression statistics are in Tables S1 and S2. Fig. S1 shows the three presence/absence datasets commencing before 1940.

Fig. 3.

Global oscillations of jellyfish. (A) Mean standardized and (B) mean logistic (binary) jellyfish indices, and (C) the probability of encountering maximum abundances of jellyfish for the global dataset between 1940 and 2011. Maximum abundance is defined as years when abundance exceeded the 90th percentile within each time series. The solid lines show a 5-y running average for annual mean (red and green) and median (blue) values. The 2 y occurring after 1940 and before 2011 are excluded from the running mean and median calculations because of incomplete data to calculate 5-y running means. Inset in A indicates the number of locations sampled over time. Error bars are ± 1 SE.