To our knowledge, this is the first study to investigate the occupational epidemiology of Danish fishery on the risk of a first hospital contact with MSDs while concurrently addressing the effect of a historical period within the trade. Surprisingly, we found no association between occupational seniority and increased risk of MSDs with accumulating years, but rather a nonlinear relationship. Our results showed that both male and female fishers had the highest risk for MSD when working less than 5 years. Meanwhile, from a lower risk estimate, the male risk showed a non-linear increase with accumulating years until reaching the highest occupational seniority of more than 20 years in the trade. Some of the decline in risk within the highest occupational seniority categorization (> 20 years) could be attributed to the healthy worker effect, where those who are the healthiest remain in the occupation (36). We carried out a longitudinal cohort study with a 24-year time span and included time-varying covariates to address the changes during the individuals’ working lives. Only two studies, covering a 1-year (13) and 5-year span (12), have previously investigated longitudinal risk of fishery work and the occurrence of MSDs. Three studies investigated the occupational seniority numerically but found little or no significant associations (14, 15, 17). A cross-sectional study from Egypt showed an OR of 1.07 for current MSD (14), indicating an inclining risk per year in the trade. That study did not, however, handle categories of occupational seniority, which could reflect the nonlinear changes of risk we found over the accumulation of years. Our study illustrates the importance of categorizing occupational seniority to see the time varying risk from fishery work on the risk of MSD, as the difference when accumulating years without categorization diminishes the nonlinear association we found.
Fishery work characteristics caused different risks for having a first hospital contact with an MSDs. The characteristics that lowered the risk of MSDs were more years in the workforce, a captain education, and working mostly part time during working life. In contrast, the risk of MSDs increased significantly when fishers shifted between trades. The healthy worker effect may be a contributing cause of the lower incidence found when having more years in workforce – because “those who work are healthier” (36). Having a captain education lowered the risk of MSDs. This is not surprising since a captain’s workload compared to a deckhand’s entails less manual lifting and more tasks related to planning and sales (13). Both status on ship – the highest being a captain – and fishing experience caused differences in responsibilities and tasks, where the least experienced fishers performed the heaviest tasks (13). Moreover, captains may also tend to work longer in the fishing trade as they are more economically tied to their ship and the trade. The healthy worker effect may be a contributing cause of the lower incidence found when having more years in workforce and captain education – because “those who work are healthier” (36). We found a significant decrease in the risk of MSDs in fishers working part time compared to those working full time. This is contrary to the reverse association seen in a North Carolina cohort study (13). However, the North Carolina study presented results from one year, whereas our results were based on 24 years of follow-up. This difference in follow-up time may partly explain the difference in estimates along with differences in country characteristics (13). Fishers working part time could either have another job that enables them to have less, or withstand, more strain. A population selection factor or healthy worker effect could have contributed to the findings concerning part-time fishers (13).
Congruent with previous studies, our study revealed that fishers are most affected by back disorders (13, 15, 17, 37-39). One cause of such a consistently high incidence of back disorders among fishers is the biomechanical load that is required to counteract ship motion. The lower extremities and low back, in particular, have an increased workload when fishers constantly have to adapt to the motions of the ship during heavy work involving work postures such as lifting, pulling and pushing (40). For men, fishery characteristics differed in terms of the risk of specific MSD body areas, although all men were significantly at risk from years in the trade. Interestingly, the high incidence of back disorders in men showed no significant association with other occupational characteristics besides years in trade. This suggests that back disorders are not indicated by these specific characteristics but are a consequence of length within the trade regardless of any other factors. Our results showed highly significant associations between years in fishery and the risk of “other pain disorders” in both genders. Future studies should incorporate these disorders as part of MSD investigations and preventive measures, as the consequences have an equally negative impact on individual well-being and societal costs.
We investigated the effect of the massive reform of fishing-quota legislation in 2002, as we anticipated that this could have an impact on number of MSD incidences in fishers. Taking period effect into account, the analysis showed no significant difference between the periods before and after the year 2002. However, including the period as a covariate significantly reduced certain risk estimates. The fishery trade has developed continuously across three decades, which might explain the modest change seen in our result. Though, these results suggest that future research should further consider the period effect when investigating the accumulation of occupational seniority longitudinally.
We carried out our analysis with age as the underlying time-exponent as suggested by previous studies (34, 35). Nevertheless, the picture of development of MSDs and their association with increased age is an important, but difficult task to fully grasp considering occupational seniority. A common problem with separating age, period, and cohort effects is the ‘identifiability problem’, whereby a specific variable can be perfectly predicted by a linear combination of the remaining variables, resulting in no unique set of regression parameters (41). In our study, it was clear that major collinearity exists when investigating age and occupational seniority. The importance of age was supported by our binary age cohort investigation, where adjusted analysis showed that men who entered the cohort at the age of 18 had a significantly reduced risk of MSD, HR 0.53(95% CI:0.45,062), when compared to those born before 1976. Thus, that a higher age when entering cohort was associated with a greater risk of first MSD aligns with the natural mechanism – that the higher age causes higher risk for MSD. Although age is not an independent risk factor for work-related MSD, older workers are more susceptible to the working conditions than are younger workers due to decreased functional capability and prolonged exposure (42, 43).
Some important methodological limitations should be kept in mind when interpreting our findings. Firstly, our study population was derived from multiple registers to ensure good coverage. As a result, we may have overestimated the selection of the study population and included more persons registered as fishers than seen in previous studies (10, 12). Data from the Danish Fishery Agency shows a lower number of fishers with an A-registration (A-registered can be owner of ship and quota) compared to our study population. Our analysis consisted of annual registrations, whenever a person was registered, no matter their registration enabling them to own ships and quotas. However, we do not believe this changed the associations shown, as we categorized by seniority. Secondly, an important caution lies again in the completeness of our case detection. Our outcome measure exclusively consisted of ‘the worst’ MSDs cases, where the fishers’ experience of pain and disability were so severe that they needed to be examined in the health care system. That the data comprised the worst MSDs cases reflects our experience that fishers contact doctors less when in pain than do those of other occupational groups (18). Thirdly, we did not investigate the influence of lifestyle (i.e. smoking, eating and sleep habits) as no data were available. Other studies have suggested that BMI has an impact on the occurrence of MSDs (44). However, populations defined by occupation are often more homogenous in several characteristics, including lifestyle choices, where e.g., job exposure matrices have been used for population-based estimates (21). We consider fishers to be somewhat homogenous in lifestyle traits to such a degree that the findings may not have been highly influenced by this. Lastly, no definitive inference can be made on the causality between the occupational fishers’ risk on MSD partly because we were not able to disentangle the collinearity between occupational seniority and age effect. The risk from several characteristics of occupational fishery coincide and add to the complexity of a causal interpretation.